To automatically render new text as revised as it's typed, simply turn on Revision Mode:
Go to the Tools icon at the upper right...
...and choose Revisions. The Revision Settings menu will be displayed:
Turn Revision Mode with the switch on the right and touch the script page to clear the Revision Settings window. Any text typed in from this point on will be marked as revised.
When Final Draft Go is in revision mode, an asterisk icon will appear on the top row of the keyboard:
To mark text as revised, select it and then touch this icon. The option to mark it as revised will be available.
To convert revised text to unrevised, select it and then touch this icon. The option to clear it from its revised state will be available.