I've just done mine, not too hard, you don't have to detach the blower. Here are the steps I used in case you haven't already figured it out, and for anyone else's benefit. I refer to part numbers / names from the Snapper 6-0376 parts manual.
1. Take the tension off the PTO belt by unhooking the idler pulley release strap (1-2977).
2. Remove the channel cover (1-3534) and it mounting bracket (1-3471).
3. Remove the clip from one end of the belt popper rod (1-3449) and pull it out (from the opposite side) as far as it will go.
4. In my case I had to remove the hairpin from the clevis pin (1-3438) on the side where the belt popper rod is no longer in the way, and back the clevis pin out a bit, because it had been inserted from the outside. I see from the parts diagram that the head is actually supposed to be on the inside - in which case it wouldn't be in the way.
5. Take the PTO belt off of its lower idler pulley on the side where the belt popper rod is no longer in the way. Move the PTO belt out of the way up above the pulleys.
6. Move the pulley end of the jack shaft in toward the PTO pulleys (i.e. loosen the drive belt tension) using the jack shaft adjusting bolt on the belt/pulley side of the jack shaft (accessed through the big round access hole, which may have a plastic plug covering it. There are two nuts, one one the inside and one on the outside).
7. Now you can remove the old drive belt if necessary, and install the new one.
8. Put the PTO belt back onto its pulley and re-apply the tension by hooking the idler pulley release strap.
9. Adjust the jack shaft to get the proper tension on the drive belt. I've found I have to adjust the tension on the chain/sprocket end of the jack shaft at the same time to get the right tension at both ends.
9. Put the belt popper rod back in place (and the clevis pin if you had to move it as I did).
10. Replace the channel cover mounting backet and the channel cover.
You've diplomatically raised an important correction to my instructions above.
In steps 6 and 9 above, the correct way to adjust the drive belt tension is using the two eye bolts (1-3445) that move the main body of the blower closer and further from the tractor by sliding it on the square tubes. You shouldn't have to mess with the jack shaft adjustment unless the chain tension is off.