I have a logitech K520 keyboard and M310t mouse connected with the unifying receiver and was able to use the SetPoint software Game Mode settings to keep right/left hand mouse button assignment. I recently had to buy another mouse and got the same as before, the M310t but for some reason SetPoint won't recognise the new mouse (something others seem to have an issue with too) and suggests installing Logitech Options which has the option to change the mouse for left handed but doesn't do anything that changing the setting in Windows doesn't already do. I downloaded both IV and V from GOG and V will play left handed. Is there anything within the game files I could do or software that would work as Setpoint did/does. Thanks for any help.

Isn't there a setting in Windows to switch left and right mouse buttons?
https://www.computerhope.com/issues/ch000819.htm

thanks Jenny. I always use mouse set for left handed and you are right that it can be changed in Windows but occasionally the setting won't be retained when playing a game. This happened more with older games but using Logitech Setpoint Game Mode forced the game to accept the setting. I had started Myst IV using my old mouse and Setpoint settings and it played left handed ok, my problem is that the new mouse isn't recognised by Setpoint and their alternative software doesn't have Game Mode setting options. I've tried the Logitech forums and seen similar issues but no solution.

If you're not getting the functionality you need, you'll have to replace the mouse.

There was a solution posted at reddit, but it's complicated, and for a different model Logitech mouse.
https://www.reddit.com/r/LogitechG/comments/6t4jq6/g403_not_letting_me_rebindprogram_left_and_right/

Have you tried uninstalling and reinstalling Setpoint? I had an issue with a different piece of Logitech Gaming software and a G502 mouse, but while searching for answers, I think I recall reading Logitech support advising people with similar issues as yours to uninstall and reinstall the latest version of the Setpoint software.

Thanks Jenny and Trail_Mystic. Yes I have reinstalled SetPoint latest version but when I start it, it only shows the keyboard and tools tabs, no mouse tab and I get a popup message 'Logitech does not support all of the Logitech devices connected to your system. Please use Logitech Options app. The Options app left/right assignments doesn't have anything specific for games. Really frustrating because my old mouse and the new one are the same model. I've started some of my older games like Myst V, Riven, Safecracker etc and they are all playing left handed ok with just the Windows mouse setting. I have waited so long for Myst IV to be updated and was so excited when it became available on GOG, isn't that always the way? I find it too hard trying to play right handed. I might try the Gaming Software and see what happens.