To remove your data (accounts and all related data), you only need to remove the application from the device. If you have several accounts, you can remove them one by one.
Unfollow Today has a beta channel that allows to test new features and bug fixes. To become a beta-tester is quite simple and there are two ways to achieve this.
1 - From the application description
At the bottom of the description, there is a button "REJOIN". This is the easiest solution and you could leave the beta channel at any time by clicking on the "LEAVE" button (still at the bottom of the description).
2 - By using the dedicated link
The second option uses a link. You need to be authenticated with the account that you use on Google Play. The same link is used to leave the beta channel: