@media screen and (max-width: 620px){
   #wpforo #wpforo-wrap {
     padding: ***10px 15px *** !important;
   }
}

Insert the code in the "Custom CSS Code" textarea located in the Dashboard > wpForo > Settings > Colors & Styles tab. 

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

Also, 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

How can I fix it so that the letters don't stick to the edges?

my web is ThaiForexTrading  Viewed in mobile view is not normal.