You can change your Viper password via your account settings. In the Viper app, just tap your avatar icon in the top-right corner of the screen and select Account.
You can change your password here at any time. Just enter your old password, followed by the new password - make sure that you click Update password to save your new password!
Please note if you signed up using your Google Account, you cannot change or reset your password because you do not have one. As you log in directly with your Google account, if you wish to change your details, please review your Google account.