Hello, @janos1, and welcome to our community!
Are you talking about changing the password for Vivaldi's webmail service?
If you still remember the old password then you can change your password on your account's profile on Vivaldi.net. Log in and click the "Manage account" button or hover on your username/avatar in the top right corner and select Account. On the profile, you'll find an option to change the password on the lower half of the page.
An alternative, which can be used both when you've forgotten your password or not, is to send yourself a password reset email. Just enter your account's username on this page, and an email with a reset link will be sent to your account's recovery email address.
For more information, please check Update Vivaldi account information.