Hi, what's the best way to replace IMAC 27'' glass when broken (latest Imac generation from end 2012) ? Thanks,

Take it to Apple is the Best Way. Any other solution will definitely VOID all of your Warranty. Which may or may not be voided already depending on how the glass got broken and whether any other Accidental Damage was done when it got broken.

Not user replaceable as far as I know. The outer glass and LCD are glued together as a unit.

The screen is glued to the machine and as said also laminated to the glass, so I imagine expensive and difficult, if you have insurance to cover accidental damage if this was the cause I would use it.

Not sure about the 2012 models but the 2010 ones can be removed in seconds, using a suction cup on the top corner. I use my tomtom to lift it away. It is attached by magnets.

Hope this helps.