In the app:
Tap on your profile picture at the top left.
Click on the blue switch account icon.
Select "Add an existing account" or "Create a new account" and log in.
Note: Once added, you can switch between accounts by tapping your profile picture and selecting the desired account.
On the web:
Go to the GETTR website and log in.
Click on your profile picture in the top right.
Click on "Switch account".
Either click on the account name you want to switch to, or select "Add an existing account" and log in with the credentials of the account you want to switch to.