help me to get 12V output from this UPS transformer GP-0915 (600VA)(240Vac)?

I cannot find out the wire connection of DC and AC connection. I have one side Blue and red wire and otherside Black+Green+Blue+Yellow.

You don't GET DC without extra parts from a transformer.

Which side has the highest resistance: that is your primary.

A UPS will output AC at the same voltage as the mains supply (depending on your locale), in your case it looks like it outputs 240v AC.

On the other hand if you take the UPS out of the equation and only use the UPS batteries, these are commonly lead-acid batteries that output 12v.

I'm thinking the OP actually has a big old transformer from inside a UPS>

Ahh yes, the perils of trying to help in the answers section on a Saturday night after attending a wedding, I missed the whole point of the question!

Yeah you'll need to mesasure the resistance across the various connections to work out what's going on. The red and blue side are probably thicker cables that would usually connect to the 12v batteries (this depends on exactly what the purpose of this transformer is within the UPS, I'm assuming it's the step-up).

But to get DC output you will need some extra bits. What are your intentions with the transformer? What are you hoping to power? or are you trying to build a battery charger?

If your hooking this transformer up to live 240v mains, be extremely careful, and if you have any doubts or uncertainty, just don't do it.