I do not have a chain whip or sprocket ring tool. Is it possible to change the rear sprocket without these tools?
To complete jm2's advice : you'll need the lockring tool. You could borrow it to your friend or go to your local bicycle workshop.
There is indeed an "ingenious technique" I've successfully used to screw the sprocket without using any chainwhip : use your chain ! It has been described here by guys from Milano, I assume you can use it to unscrew your sprocket :
For the lockring a hammer and screwdriver works sometimes, but you end up damaging everything involved except the hammer, sometimes to the point that you can't get the lockring tool to work after the fact.
Get the lockring tool. There are semi universal ones that work ok. The type that are designed to fit specific lockrings (ie 3 slot, 4 slot) work much better. Either way they're fairly cheap.
Once the lockring is off you might be able to use your bikes chain and crank in place of a chainwhip, but this is yet another place where you're better off just buying the appropriate tool.
Once you have them you'll be a happier fixie rider.
Does it have a lockring? If not, just get a friend to hold the bike steady and try to pedal backwards.
I don't have a lockring on my commute bike and, after a series of braking manoeuvres in short succession, I've had my cog come loose while in action. That's quite scary!
If you do have a lockring then, well, you can do it with a hammer and screwdriver - it's really just a question of whether you want to reuse it again.
Or just wander into your LBS and ask to borrow their tools. Most good mechanics will gracefully decline, but will explain to you why buying one yourself is the best solution ...
With two wood blocks in a vise or drawn together by long bolts, you can grip a sprocket without damage. In the years when I placed a sprocket on my road hub (no lockring, even though a bottom bracket lockring would fit as a jam nut), it finally was on so solidly that I could skid the rear wheel without breaking free.
I seem to recall that in desperation one day, since it was a fixed gear bike, that I left the chain on the rear sprocket, while off the front sprocket allowing me to wrap the slack around the BB and secure it somehow. Then I could rotate the rear wheel forward by hand, which was able to release the sprocket.
A home-made chain whip is not too hard -- metal about 1/8" thick, drilled to allow a chain pin through. I don't think I added a length of chain for sprocket support, but rather let the 'whip' chain wrap around the sprocket. The metal piece had to be reinforced with wood to prevent buckling. Jim Papadopoulos