I have game updated to 1.1.5.10. Current version is 1.1.5.12, There is also patch from 1.1.5.11 to it in the download section, but it does not apply on 1.1.5.10 and there is no 1.1.5.10>1.1.5.11 patch. How to update (installing GOG.Galaxy is not acceptable for me)?

Considering that you don't use Galaxy and that there is not a patch for the version installed on your system, you probably must uninstall the game, download the updated installer and then install the game again.

Other users already wrote posts about the missing patch in the changelog thread, but you may want to directly ask the GOG support or the game developers about it.