There are two great widgets for this in the app store:
1) https://appstore.home.mendix.com/link/app/108281/
Lets you define an event for any change to an object. Use it to save changes when they occur.
2) https://appstore.home.mendix.com/link/app/21721/
Shows a message when the user tries to navigate away from a page. Use it to remind users that they have unsaved information.
-Andrej
Create a Save Entity Microflow and attach it to the On Change event of the field being editied.