Just tap the account logo of the account you want to rename on the home screen, and then tap on it again, and you can edit the name of your account by selecting the pencil icon next to your existing account name. Simply make your changes and save and you're all done.
You can also do the same through the 'Settings' screen. Just tap the cog icon, then 'Manage Accounts' and tap the 3 dots beside the account name you want to rename.
The following short video shows you how easy it is!