First change your key password in Mailvelope. Then you should create a new recovery document to use the new key password on other devices as well.
Open your browser with the installed mailvelope-addon and follow these steps:
Create a recovery document to be able to restore lost encryption data and/or use it on other devices.
Lost your recovery document? Follow these steps to create a new one.
Keep your recovery document in a safe place for future reference.
If you have set up Encrypted Communication on yout computer and have changed your key password via mailvelope, you will have to synchronize your GMX mail app for Android with the new key password.