You can change your email account password on your provider's website — this isn't something you can do from within Spark. Once changed, the new password will apply across all your devices.
If you change any other account credentials (not just your password), you'll need to re-add your account in Spark for the changes to take effect.
Here are instructions on how to change the password for the most popular email providers:
When you open Spark after changing the password, the app asks you to log in again to update your account credentials.