You can switch it in the config file - SpudTabletArmSettings. You can open it in Notepad.
If you open that, there's a section called [Modes] and under that has;
;To make Spud Arm left-handed, you need to set LeftyMode to true and then Transform -> Flip Horizontal the captured Spud Arm source in your streaming software.
LeftyMode = false
change the word "false" to "true", save the file, and then in OBS (or whatever you use), just flip the window source. That way it'll be left-handed without your pen being on the wrong side of the tablet while you're drawing. Hope this helps!