Method 1.
type autoexec.bat > autoexec.new
type yourfile.bat >> autoexec.new
ren autoexec.bat autoexec.old
ren autoexec.new autoexec.bat
Method 2.
add this line to the end of autoexec.bat using EDIT
yourfile.bat
In the above examples, yourfile.bat means "the name of your batch file"