Quick question, how are you invoking the SSMS, by clicking a menu item or short-cut?
If so, right click on the menu item/short-cut, select Properties and Open File Location. Then right click on the SSMS.exe and select Properties. The Details tab will tell you if it is the right version and obviously if it does not find the target, then you know what is wrong;-)
If you are invoking it by pressing the Win/start button and typing in SSMS, you can right click on the SSMS icon when it appears, select Properties and Details.
I find that wrong/obsolete links are often the issue after upgrades.