1. Follow the steps in this video:
Samsung Galaxy S5 : How to clear cached data for all apps (Android Phone) - YouTube
2. Or try this free app: Turbo Cleaner - Boost, Clean, Space Cleaner. This one is from Apex Apps. There are many available, so look for the one from Apex Apps.
I use it on an older Android tablet which has Android 6.0. It may not work with Android 7.0 and higher which I don't think you have on a Galaxy S5.
Good luck!