First reboot your device.
Make sure your device is connected to internet.
Now you might get a notification that marshmallow update is available.
Just download it.
If you didn’t receive any update.
Then you have to download it manually.
Go To setting>About Phone>System Update.
In System Update you will see ‘ Search For Update’
Tap on that.
Now your device will search for Marshmallow update.
If the update is available it will download automatically.
After downloading  reboot your device.
We would also recommend you to use high speed internet.