My DM will only try to install the highest OS on the computer or from the internet. If I want to select any other OS, I have to delete files etc.
Can I change this? If you could help I would be grateful
The usual deal is to uninstall any OS that you are not installing to your phone. You don't have to deleted the actual installer package from your computer just not have it installed (trying not to be confusing). Also you should disconnect from the internet while installing your OS of choice. OR don't use DM and just run the loader program.
