Welcome to the forums, Kuro
Please be aware that this is specifically an editing site, so we don't do gameplay tips. However I'll give you the benefit of the doubt this time and assume you want the scripting solution rather than the gameplay solution...
To get a unit to drop it's main weapon you can use this line:
unit action ["DROP WEAPON", unit, 0, 0, (primaryweapon unit)]
hmm...
ok ok... the gameplay tip method:
Go to the map screen and click on the gear section of the briefing. Move through to the guy you want to drop his weapon, and click the drop weapon button (looks like an arrow pointing down).
You can order a soldier in your group to pick up a weapon by using the action section of the radio menu (6).