With each release we publish instructions on how to upgrade
Now we have released an upgrade tool, see
This is the only supported method - if you "upgrade" to the next version of Ubuntu using Synaptic or apt-get we do not support this as you risk breaking your system completely. This means no questions answered about it.
If you fail using the recommended method you are welcome to ask for help
Here's a link to a topic about this when Hardy was the newest
http://www.linuxmint.com/forum/viewtopi ... 43&t=12311
Here is a link to upgrading from Felicia (Mint 6) to Gloria (Mint 7)