You cannot change a password using Aqua Mail. We do not store or access you passwords and cannot change or restore them for you.
You need to open you email provider's website in a browser and follow their designated steps for password recovery. After that, you may have to submit your new password in your Aqua Mail account if the synchronization stops working.
To do the above, please go into the Aqua Mail App > Settings > Manage accounts > your email address > Account Setup. On this page, you can edit your password and re-authenticate the account by pressing Next.