How do i free up pinion bolt in a chevy rearend?

Use a bar with holes in it to attach to the pinion flange
that will allow you to hold the pinion while undoing the nut
use an impact gun that has a capability of more than 400 ft/lbs

You need a 250lb torque electric impact gun to ever think about getting the pinion nut loose. It really doesn't take that much power, but the bigger the better. You also need to fix the tires so they do not turn while you getting the nut loose, otherwise the rear will just turn the tires. Only reason I figure your trying to get the pinion nut loose is that you want to change or reset the differential gears. You're best off taking it to a shop. If you don't know how to get the pinion nut loose, You're not going to understand how to do the rest of the process either. Take it to a shop or dealership Bro.