I just bought a 79 kz 650 and I was told the carbs need reset. How do I go about doing this?

You were told the carbs needed "reset," told by who?
There's almost no information here, you will need to say a whole lot more about what's going on. "reset the carbs" means nothing.

Did the machine come from a place where the altitude is radically different? IS that why? Or did the previous owner customize it?

If the PO had non-standard pipes or air filters on it, he'd have had to have altered the jetting in the carbs. If he sold it to you without some of the custom stuff, it would need to be set back to a more factory configuration to run well.

Does it run? Does it run pretty good?

If it does, try a plug test. Get a set of brand new spark plugs for it. Set the gap according to the manual.

Run it at fairily high speed for 30-60 minutes to make sure it's warmed up-- on the old plugs. The change to the new plugs and run it down a long piece of road where you can coast to a stop safely. Get it up to a good speed and kill the engine while the revs are still above 3500 to 4500 rpm. Coast to a stop and get out of traffic. Remove all the plugs and look at them in good light. If they are burned white, you may need a different plug or the carbs may be much too lean. If they are black, sooty, oily, especially if they smell strongly of gas, the carbs may be too rich. If they are a nice light tan color, the mixture is probably pretty good, especially if the machine runs well.

-T