guilhermemuck
It's summer time here in Brazil and while playing some games @ 4K res my GPU often goes to about 80C while keeping clocks of 1900mhz on the core and 7000mhz on the memory.
 
Unfortunately I don't have air conditioning in my house so I am considering doing a small undervolt on my gpu to reduce the heat generation for a while.
 
How can I undervolt it using PX1?

You could use the temp target and it will do all of that for you instead of creating instability with the undervolting because if you do that, you need to bring the clocks down to stabilize the undervolting.  Temp target does all the work for you so lower it and go from there. 
 
Btw, 80c isn't so bad for being on air w/out AC since the GPU can handle up to 89c before it enters a thermal shutdown state to protect itself so you shouldn't worry too much.