Yes! There are two ways how you can change your Webmail password.
1) You can change it in Control Panel -> Email Accounts.
In there, search for the Manage E-mail Accounts section, select your email account and choose Change Password to change the account's password.
2) You can change your webmail password in RoudCube webmail itself.
Simply login to your Roundcube webmail and go to Settings -> Password. You will be able to change your password there.