You can rename your town in two ways:

Using the town sign
1. Find the sign with the current name of your town (by default it’s near the train station, unless you moved it).
2. Select the Pencil icon.
3. Enter a new name for your town in the pop-up window.
4. Select the Tick icon to save the new name.

Using your player profile
1. Select your avatar on the home screen.
2. Select Change, then click the Pencil icon to edit.
3. Enter a new name for your town and confirm to save it.