From https://www.androidpit.com/how-to-clear-the-cache-on-the-galaxy-s6How to wipe the app cache on the Galaxy S6
If you want to clear the app cache on the Galaxy S6 – whether for one app in particular or for all apps, you do this through the Settings menu. Go to Settings > App Manager and find the individual app of which you want to clear the cache. Remember it might be in the Downloaded, Running or All tab. Tap the entry and then tap Clear Cache. If you want to clear all app caches simultaneously, go to Settings > Storage and tap Cached Data > OK.