Any way to lock my bootloader again (H815 eu)?

I unlocked my bootloader to try CM13, but I'm not happy. Now I can't find any guides or pages that explain how I can revert my phone back to original state (bootloader locked)? Can't sell this phone when the device unlocked text is on the boot screen :(

Plese don't respond if you just tell me to google or search, because I did that...