Found old wallet… how do I use it?

Hey!

I remember buying between 7 and 10 “Darkcoin”.
I then backed up the .dat file and forgot about it.

I just found it, how do I use the wallet? I have forgotten pretty much everything as it was so long ago!

Reading the forums I think I first have to upgrade the wallet (I tried downloading electrum and opening the wallet and it failed) so I figured I would ask here for the steps instead of fiddling around and failing.

Thanks!