move and activate wordpress from subfolder to the root domain

Hi, I would like to kindly ask technical advice:
I installed in a sub-folder a new wordpress
already exists in the root installation up and running
I should now make the main installation in the sub-folder, but I would not erase what’s in the root to avoid any problems
how do I use it with the main domain and disable the old installation
thank you very much
Gianluca (sorry for my english)