If you need instructions on how to access the terminal please look [here]. Please Note that file names in Linux are case sensitive. If you think you may want to revert there is a backup utility in the My OSMC add-on. The following will remove just the video database and cached artwork leaving everything else in your setup intact. You sources will still be present but you will have to setup the scrapers again as this information is stored in the database your deleting.
systemctl stop mediacenter
rm -r ~/.kodi/userdata/Thumbnails
rm ~/.kodi/userdata/Database/Textures13.db
rm ~/.kodi/userdata/Database/MyVideos116.db
systemctl start mediacenter
And just for reference if you wanted to just remove your entire Kodi setup and start from scratch then it could be accomplished with…
systemctl stop mediacenter
rm -r ~/.kodi
systemctl start mediacenter