Generally you should just run the new installer. It will override the old application/plugins and keep your data. But to answer your question, delete the app, plugins and the data folder. More precisely:
/Applications/MIDI Guitar 3.app
/Library/Audio/Plug-Ins/Components/MIDI Guitar 3.component
/Library/Audio/Plug-Ins/VST3/MIDI Guitar 3.vst3
~/Library/Group Containers/X6D2Q4W5A8.midiguitargroup/
The latter is a folder, a so called “app group”, which is given by Apple and may hold shared data also from other Jam Origin products (in the future). It contains your user data related to Jam Origin products (patches, plugin lists and settings).