Before uninstalling the Final Draft Mobile app from your iPad or iPhone, use iOS's Files app or iTunes to back up all of your scripts to your cloud storage service or your computer.
When you remove the Mobile app, your scripts will be deleted from the device. Back them up first.
To remove the app, touch and hold the FD Mobile icon and choose Rearrange Apps:
Next, touch the X next to the icon...
...and the app will be removed.
To reinstall. log into the App Store with the same credentials with which you purchased the app originally, do a search for "Final Draft Mobile" (without quotes) and choose Install.