What exactly do you mean by “manual install”? Is it a truly manual install (meaning you download the zip file and extracted that to some folder) or do you mean you just used apt instead of using openhabian-config (which it’s worth mentioning uses apt to install openHAB).
If the latter OH will update with a standard apt update and apt upgrade. If the former see openHAB on Linux | openHAB (a couple sections further down in the docs from those linked to by @justaoldman.