Did you keep you're original P5 Runtime (I hope for your sake you did).
If so then:

Uninstall P6. Make sure that the whole of the P6 directory has gone (delete it if necessary).
(possbly not necessary, but will save you time if it does turn out to be necessary..)

Re-install P6.
Open P6 and go to (any of) the libraries.
Trudge up the arrows til you get to the top. (You'll know you've got there when you see a panel with a big red dot near the top).
There you should find the option to add a different runtime to the existing one.
Click on "add new runtime" (I think that's what it's called), navigate to your P5 runtime and add it.

You should now be able to switch between both...

Cheers,
Diolma