If you have an older account which isn't connected to the Portal, you can click on the cogwheels below your current level (top right hand side) in-game and you'll get to your personal game settings.
Here you may select an avatar, turn off the sound, change your password or delete your account.
If you have a Portal account (or just a newer account in general), you can change your password in the Portal here: https://en.upjers.com/settings - under the password tab!