You don't have to overwrite the default 737, you have to create a different folder with default plane files in it and then in that new folder overwrite with zibo's file. Tha way you have both, the default and the original aircraft to choose. If you hover your mouse pointer over the plane's tile in the x-plane ui it will show you the address/folder the acf is coming from, that way you make sure what version will you choose.