First you need to log in to the Self Service Center at https://user.disroot.org and choose to change password.
Once you changed your password in the Self Service Center you will need to make sure to change your encryption key in Nextcloud, since all your files in ownCloud are encrypted with a key that is generated from your password.
Invalid private key for Encryption App. Please update your private key password in your personal settings to recover access to your encrypted files.
TIP: If you lose your password, you won't be able to retrieve your files on Nextcloud as they are encrypted so that even the server administrators can't see their content. If you want to be sure you won't lose your data check the Enable password recovery box in your Personal settings. This will allow the server administrators to retrieve your data upon request.