Can't open mail after update

I am getting an error message saying that mail can't open because the newer version may be located elsewhere than in applications. I have checked and I have versions 4.5 and 4.6 in applications. I have followed instructions in help to reinstall Mac OS and security update then try mail again, but it still doesn't work. Do I need to delete 4.5? Sounds daft to you experts I'm sure, but am worried I will lose all my emails if I do so. Also, seems that Adobe wants to open all files as default, but I can't figure out how to change that (in case that's the problem). Would love some help, please.