How do i replace the upper control arm bushing on my 2000 GMC 4wd jimmy blazer

Use a C clamp type bushing tool to press out the old bushing and install the new one.
Alternately, you can use a pneumatic chisel to dent the bushing from the back being very careful to not nick the control arm or the shaft. A little penetrating oil would help. Once the bushing loosens the pneumatic chisel can pop it out.
Mark the shaft with a piece of tape to get it back the same way. Once the first bushing is out, the shaft should slide out through the hole making it easy to remove the second bushing.
To install a new one make sure that the A frame won't bend when installing the bushing. Don't forget to put the shaft back before installing the second bushing. Don't tighten the nuts on the ends of the shaft until the A frame is installed and lower A Frame supported with a jack approximating the normal ride height. Fixing the bushing rotation at normal ride height will make them last longer.