I have a 2005 Chevy Avalanche that will go into drive but won't go in reverse.How can I fix the problem

Unless you have mad skills, you won't be able to fix it. Most likely, either the reverse band has gone out or the servo/plunger which operates it is no longer doing its thing. Either way, you'll need to take it to a shop to have it fixed. Reverse band is pretty much a rebuild. Getting the plunger fixed is hit or miss, but can be done by itself.