Go into edit the form that will send the data and trigger the entry update.
Click on 'Settings' → 'Actions & Notifications' → 'Send API Data'. This will add an API action to your form.
Insert the following in the Notification URL box:

yoursite.com/wp-json/frm/v2/entries/25

Replace yoursite.com with the site that holds the entry you would like to update. Replace 25 with the ID of the entry to update. You can

yoursite.com/wp-json/frm/v2/entries/[display-frm-data id=x u_id="[y show=ID]"]

Replace x with the ID of the View and replace y with the ID of the userID field in your current form. This would update the first entry

Make sure to use the receiving field key in the "Key" column. Use the sending field key or ID in the "Values" column.
Select 'PATCH' as the Method. This will ensure that only the posted fields are updated.
That's it! Now try submitting an entry in the sending form and see if it updates the entry in the receiving form.