engine problem!! need help

well when i'm punchen it in 4th around 6000rpm's i start to smell i think burning coolent and my friend thinks that there may be a crak in the block but i think a may have blown a head gasket becuase it has white smoke coming out of the muffler when i start it and it only does that when it's around 20-40 degress outside.

if it is the head gasket how hard is it to replace it?