You can change your webmail client on Plesk using below mentioned steps:
1) Login to your Plesk Panel and Click on ‘Mail’ form the primary menu.
2) Click on ‘Change Settings’.
3) Under Settings section, you will see a drop down menu next to Webmail.
The default webmail client set is Horde.
4) Select the option ‘None’ from the menu and click OK
5) Click on ‘Change Settings’ again.
6) Under Settings section, select the option ‘MailEnable WebMail’.
7) Hit OK to save the changes.
That's it. Now you can access your email on your choice of webmail client.