"Thanks. I have 4GB, should be ok??
Do I install over, or full new install??"
I've got a 2012 Mini with ML, and 4gb of RAM is doing fine for what I need (your needs may be different).
If you have a way to completely backup the internal drive (I'd suggest a bootable clone created with either CarbonCopyCloner or SuperDuper), I'd suggest
- backup to external
- boot from external
- re-initialize internal drive
- install new OS
- migrate old account and data files over with migration assistant
That's how I'd do it, others might suggest different approaches.