Originally posted by u/en_rov on r/AmazfitBip.

Hi everybody. I recently posted some videos of BipOS, the "calendar/calculator/torch" custom firmware to the Bip. Since I saw somebody was interested in knowing how to install it for themselves, so I decided to put together this guide.
DISCLAIMER: As you can see from this ( https://www.reddit.com/r/AmazfitBip/comments/devz74/oc_video_showcase_of_bipos_running_on_my_watch/ ) video of mine, I'm running the custom firmware (cfw) myself. Despite this fact, I'm not responsible to any damage to your hardware. I personally flashed back and forth from stock to custom, downgraded, and never had an issue, but your mileage may vary.

Background:

First of all, I must point out that Bip firmware come in two flavours:
- Latin - For Russian, Spanish, German, Italian, French, Turkish languages
- Non-latin - For everything else
Firmware components can be swapped out only if they are of the same flavour. Bip firmwares are composed by three files:
- *.ft - font
- *.res - resources (applications, icons)
- *.fw - firmware core
Now, custom fonts and resources exist, and can be swapped around provided that they are of the same flavour.
Lastly, custom firmwares can't be used with the official Mi Fit application right now, since it looks for updates for the watch firmware and installs the latest stock version if it finds that the installed firmware is older. From what I've read, a fix is possible, but as of right now you should stick with third party applications like Notify and Fitness or Gadgetbridge.
Guide:
Now with the theory out of the way, let's get to the actual guide (Android only):
1. If you're using Mi Fit, you should uninstall it before starting the update process. As already mentioned, you can't use it and keep a cfw at this point.
2. Make sure both watch and phone are more than 50% charged.
3. Navigate to the source forum ( https://myamazfit.ru/threads/bip-mnvolkov-bipos-en.1087/ ), make an account, click the "download" button, and download the "Full set.." archive depending on which firmware "flavour" (latin or non latin) you want. In my case, I chose the " Full set MNVolkov BipOS v0.5 based on non-Latin 1.1.2.05 (official firmware, SantaX MOD) " version.
4. Unzip the file on the phone, install Gadgetbridge (https://gadgetbridge.org/) or Notify and Fitness ( https://play.google.com/store/apps/details?id=com.mc.amazfit1&hl=en )or any application that lets you flash stuff to the Bip.
5. (Fix for Russian only Alipay menu) Replace the firmware file you find in the Stock_1.1.x.xx_*latin" subfolder of the unzipped archive with the corresponding version in this archive ( https://anonymousfiles.io/sfQOcJDY/ ).
6. Depending of the app you choose, you will have to follow a different procedure:
Gadgetbridge:
1. Pair the watch to the application.
2. Using a file browser, go into the Stock_1.1.x.xx_*latin" subfolder from the archive, depending on which version you chose at step 2. It's "Stock_1.1.2.05.not_latin" in my case.
3. You will find the three firmware components (see the introduction of this guide) in there, tap on the font file, it should open Gadgetbridge and prompt you to install it. Tap the button.
4. A progress bar will appear, and the watch will display an update screen.
5. Once it's done, the watch may show an error screen, "connect to the APP to update". That is normal.
6. Repeat the steps for the resources file, and then the fimware file, in this order.
7. After the firmware has finished, the watch will reboot.
8. You're done. From Gadgetbridge you can enable the "Alipay" menu option, which is used as mod menu.
Notify and Fitness:
1. Pair the watch to the application.
2. Navigate to the rightmost tab, with the two wrenches, then  "Tools". Tap the "Update Firmware" entry, then "Custom Firmware".
3. A file browser will open, go into the Stock_1.1.x.xx_*latin" subfolder from the archive, depending on which version you chose at step 2. It's "Stock_1.1.2.05.not_latin" in my case.
4. You will find the three firmware components (see the introduction of this guide) in there, tap on the font file, it should go back to N&F and prompt you to install it. Tap the button.
5. A progress bar will appear, and the watch will display an update screen.
6. Once it's done, the watch may show an error screen, "connect to the APP to update". That is normal.
7. Repeat the steps for the resources file, and then the fimware file, in this order.
8. After the firmware has finished, the watch will reboot.
9. You're done. From the "band settings" menu you can enable the "Alipay" watch entry, which is used as mod menu.
Here are two YouTube guides for a visual walkthrough of the update process on Gadgetbridge ( https://www.youtube.com/watch?v=UKzuPu-Jq-c ) and Notify and Fitness ( https://www.youtube.com/watch?v=ASdtUf1bqcw ). The N&F video is outdated (the menus are displayed differently) but you should get the jist of it.
If you have any questions, feel free to ask - after you've read the whole guide twice.
EDIT: I checked back the original thread, with mod version 0.5 there's an issue where the strings for the application names in the "Alipay" menu are only in Russian. I added step 5 to the guide to fix this issue, if you already followed the guide you can just flash the fixed firmware over and you'll be done.