This one is pretty easy! Just download the Service Pack i.e. AutoCAD 2013 SP2. Type in the following commands in a command prompt:
NOTE: C:\path\of\your\downloaded\SP = The location of your Service Pack
If you want to convert an .exe to a .msp so that you can add it to a deployment, you need to run this command:
AutoCAD_2013_SP2_64bit.exe /e ./AutoCAD_2013_SP2_64bit.msp
Thanks to Alison Gangler for sharing this tip to the community.