To rename a custom folder, follow these steps:
Click on the My Library tab (second icon at the bottom of the screen beside the Home icon).
Select the folder you would like to rename. (If you are unable to see the folder you are looking for, click Edit on the top right corner of the screen to add the folder to the shortcut list.)
Click the three dots on the top right corner of the screen.
Click on Rename folder.
Input your desired folder name.
Click on Save.
Your custom folder should now be renamed to the new name you have given it.