Hi,

If you want to change the position of the bubble only on mobile devices please use this CSS code: 

@media screen and (max-width: 600px){
       #wpd-bubble-wrapper {
             bottom: 300px;
       }
}

If you want to move for all kinds of devices, then this CSS code should be used: 

#wpd-bubble-wrapper {
       bottom: 300px !important;
}

The red-marked value can be changed as you like.

Insert the code in the "Custom CSS code" textarea, located in the Dashboard > wpDiscuz > Settings > Styles & Colors admin page.

Don't forget to delete the caches before checking.

Additionally, please try in incognito mode if you're checking on mobile devices. Because mobile browsers have a hard cache, you'll not be able to see the change immediately.