You must load it before the Sandbox module if you created a new module, or the other more direct way is, edit the original xml settlements.xml inside sandbox and make a backup in case you want to reset
I tried just now and I have override the towns owner and prosperity, but the town’s name remains the same even though I changed it.
Edit: I have override the town’s name when I load it after Sandbox, but not the owner, while loading it before Sandbox will let you override the owner, but not the name.
Changes only takes effect after a new game, so if ever you edited the original settlement.xml, starting a game with it and save, and restoring it to original file will still let you keep the owner of town the last time you saved.
Though changing the town’s name can be done anytime.