Backing up your account is simple. You may well have already done it without realizing it.
If you used google play or game center when you first installed Rival Gears, you are likely to already have your account backed up to one of those services. Alternatively, if you signed in to Facebook or Twitter after installing rival gears, you are likely to already have a back-up of your account on one of those networks.
To make sure your back-up is up to date, or to back-up an account that has not yet been backed up, enter the "Player Details" screen by clicking your player name in the top-left of the screen. On this screen you can ensure that you are still logged in to the Google play, Apple Game Center, Facebook or Twitter account that you previously logged in to, or you can log in to one (or many) of these accounts to create/update a back-up.
When updating a backup, be sure that you select the correct option! Selecting "RESTORE" will recover a previously saved racer, while selecting "OVERWRITE" will overwrite your previous saved racer with your current racer.