How do I reset Safari to default?

As the "Reset Safari" option is no longer available, the best you can do is go to Preferences --> Privacy --> Remove all website data. Then, while your Preferences pane is open, go to the Advanced tab and click on "Show Develop menu in menu bar." Once you see it, click on Develop --> Empty Caches. Go back to your Preferences, click on Security, and make sure "Allow Plug-ins" is unchecked. You can then go in and uninstall all your extensions, clear all your bookmarks, and anything else you've added to it.

Also try going to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J. When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder. Select Library./Caches/com.apple.Safari/Caches.db and move it to the trash.