You can change it within the language definitions- you just need to be in the correct place.
Under System > Localization > Language Definitions
Type account_create in the Block, then enter that will give you all the account creation pages xml fields.
You want entry_city
click on the Edit icon on right and change the field to City/Province
SAVE
When you are using this for other changes be sure you change box is STOREFRONT if that is where you wish change to be made. There are also Admin changes for some thiings.
Lee