I suggest you to back up your iPhone 6 data to iTunes first, then you go to "Settings > General > Reset > Erase All Content and Settings" to reset your iPhone back to default. After that, restore the backup from iTunes.
It will reclaim much free space by this way. And most of your important data will not be lost. It will take much time to back up and restore backup, so please wait patiently.
Or you can also consider an iPhone cleaner, like iMyFone iPhone Cleaner to thoroughly remove useless junk files and hidden temporary files from your iPhone 6. It will be easy and not time-consuming. You can google it and have a free try. It will also losslessly compress your photos, backup your photos or videos to save space and more.
Have a shot.