Upgrade to a recent version requires a license key that fits the release nummber. So you could try to do a repair via the configuration menu on your PC first. Write down you MM6 key so you can re-use it if necessary.
If this does not work it may be that your PC has an operating system version where MM6 does not work. In that case you should buy MM15 (and get MM(20-)16 for free with a lot of extra's you didn't know you missed until you used them. ;-) MM6 is way too old to get an upgrade price.