Do I uninstall the old version before installing the new one ? if I install the old version then what happens to my wallet and the funds in it? how do I restore the wallet?
If you have the 12-word seed with you, there shouldn't be any problem. Just uninstall the program like what you would normally do with other programs and then reinstall it back. Look under File menu and click New/Restore. Choose a file name and then save it. The program will prompt you whether you want to start with a new wallet or restore one from a seed.
As in your case, you should select restore from seed