Please follow the below steps.
From the main calendar screen in TimeTree,
- Tap the main menu button in the top left hand corner of the screen
- Select the calendar you wish to rename
- Tap the "Settings" tab in the bottom right hand corner of the screen
- Tap on the name of the calendar below "Calendar Name" and type the new desired name
- Tap the checkmark (✓) in the top right hand corner of the screen to save your changes