Ahhh, here you go with mods!
The owners manual has the torque spec for the rear axle only for removing a flat tire, but not the sprocket bolts. Limited info but good to read though.
For the more involved maintanence you're going to need a Service Manual (Seek/search and you will find).
IMPORTANT: Just like cars, there's a lot of "mechical wisdom" the service manual won't tell you.
To help you more, it'd be good to know why you want to replace it. Going for a ratio change, Is it wore out, wrong color? It's all good!
If there's something mechanically wrong with it, may have to look into a complete fnt/rear/chain replace?
Sound advise about a torque wrench. I use a good quality "beam-type" they're not horridly expensive, but does the job for me.
The biggest troublemaker you'll probably ever have to deal with, watches you from the mirror every mornin'.