Certified GKH Gardening Expert
We would not recommend pruning an avocado that much, especially when it is that small. Right now, you can take maybe 2-3 inches off, but any more than that will likely kill it. You don't want to prune more than 1/3 of a plant in a year.
To prune it, just use a sharp pair of scissors to snip off the part of the plant you wish to remove.