CURRENTLY as explained in the release notes Peppermint 10 would require a fresh/clean installation (not even keeping a /home partition).
In due course instruction for MANUALLY upgrading Peppermint 9 --> 10 will be posted on this forum, but ONLY when they've been FULLY tested, then tested again.
But..
We will not be providing in-situ upgrades as an automated process because in-situ upgrades ALWAYS come with risk, and they encourage users to attempt them without backing up. A fresh install forces users to consider what they NEED to keep and ensures they back it up first.
Posting instructions to MANUALLY upgrade will also be easier to support, the user will already be on the support forum (because that's where the instructions will be posted) and as a clear series of steps, should something go wrong, it'll be easier for us to figure out precisely where (at which strep) it went wrong and affect a solution. This has to be a consideration where support manpower is at a premium.
I should also add that sure, where we push users to 'fresh install' any data loss could never be considered 'our fault', yet any data loss incurred via an automated process certainly WILL be considered 'our fault' .. so there is a certain level of self protection going on here, but it's selfishness aimed at protecting the user as much as us. As i said, there is ALWAYS a small risk, and with limited support manpower available to us it makes perfect sense for us to keep this risk to an absolute minimum, and therefore be able to offer support in the few cases where things don't quite go as expected.
I'll get around to the instructions in about a week, when things quieten down a bit .. thanks for your patience and understanding

Forum Moderators & Trusted Users - This is now the official Peppermint stance, so can be copy/pasted whenever this question is raised.