You can change one shock at a time,but going to longer shocks your going to need some form of jack. do you have a car jack? can you borrow one from a friend? do you have any friends?  Can you hang the bike from something overhead? can you set the frontend of the bike on flat boards ,stake it down and dig a hole in the ground under the rear tire ? do you have determination ?

You could lock the front brake buy wrapping tape around the lever and grip , ,and tie the bike off to something on both sides for balance,then place the jack under the motor(not the swing arm it needs to move ) jack to take tension off the shocks ,remove, jack a bit higher ,install the new shocks .

And if you don't have a tie off on both sides, if you have a post or small tree you can roll the front wheel up against it and just strap the front wheel to the post. That will offer some security to keep the bike from tipping left or right, but still allows you to jack it up because the wheel can rotate on the axle (if the front brake isn't on at the moment).

I used a log, lifted the rear fender against the rear tyre.
Worked fine.

Actually, I made a stand in just a few hours last week.  Used two pieces of 2 x 4's and some 2 pieces of 1/2" plywood, and a piece of 3/4" water pipe.  Glued the pieces of 2 x 4" together and sandwiched them between the two pieces of plywood.  Let dry.  Approx. size:  about 18" long by 7 inches tall.  Then drilled a hole in the left side to accommodate the water pipe.  Then sort of rounded off the top so it would "roll" better.  I take the bike in my left hand and position the stand under the rear of the motor area, and tip the stand into position by using the pipe.  When the pipe right end hits the ground, the weight of the bike holds it in place and the bike is up about 1 or so inches off ground.  Putting the stand more forward raises the front end more, and placing it further back does more to raise the rear end!

I'm pretty cheap and this worked better than I thought.  Cost me about $10.00