Right click on the Sins icon to get the uninstall option.
You can actually keeo both 1.09 and 1.05. Simply copy your entire 1.09 Sins folder to another location (copy, not move). Then uninstall it, have "show pre-release" unchecked, and install 1.05 through Impulse. The 1.09 copy that you made will run fine from the other folder, and you'll now have 1.05 as well.