Open the DefaultWeapon.ini located in the directory "C:\Program Files\Electronic Arts\Medal of Honor Airborne Demo\UnrealEngine3\MOHAGame\Config" with Notepad.
Press Ctrl+F and enter "ExpPointsLevel1=" without the quotation marks. Now change the values for ExpPointsLevel1=, ExpPointsLevel2=, and ExpPointsLevel3= to 1, 2 and 3.

So instead of this:

ExpPointsLevel1=25
ExpPointsLevel2=50
ExpPointsLevel3=100

it should read like this:

ExpPointsLevel1=1
ExpPointsLevel2=2
ExpPointsLevel3=3

This way you can get all the weapon upgrades after 1, 2, and 3 kills instead of 25, 50 and 100 kills. Do this for each weapon so you can see the upgrades available. In the demo you are only able to use the Colt 1911, MP40, Mauser K98, Thompson sub machine gun, and the M1 Garand.

Note: This code applies to the DEMO version only.