No, not reinstall. You can clear the cache by going to the settings menu –> app settings (on my S7 it’s called “Application Manager”, may be a bit different on your phone). There, scroll down until you find Magic Dosbox in the list. Tap it once, and in the next menu, tap “Storage”, then “Clear Cache”. That’s it for Touchwiz Marshmallow, at least.
I see you’re still on Lollipop. It’s possible that the “Clear Cache” button will show up immediately when you tap on Magic Dosbox in the App Manager.
Do the same for your launcher, just to make sure.
If you can’t find the option anywhere, look for “clear cache” on Google Play, there are plenty of apps that can do the job.