First
If your file name is like this Filename.rar just replace "rar" with "avi" (if "hide extensions for known file types" is dissabled)
else
open command prompt then browse to that folder
then issue this command
"ren filename.rar filename.avi" (without " ")
Simple
and if "hide extensions for known file types" is dissabled then i suggest you to enable it.
how? here are the steps
open any window/folder
click on tools from menu
click option "Folder Options.."
choose "view" tab
here you will find "hide extensions for known file types"
enable it if it is dissabled
thats it.