heya mate i know what you mean cause i have just quit playing CO2 as well and started lately to play SRO, much better graphics and u don't have to spend all your life jumping as a monkey like in CO2 :D
so as guys here told you, there is not anyway to upgrade level of anything in this game like in CO2 .. you got to use that lvl 16 SOS weapon till you reach lvl 24, then to sell ur weapon and try to get another lvl 24 SOS weapon, and that's how it's going with all other items in the game .. btw, if you got some item in ur stuff which is (for exalmple) lvl 16 not SOS, SOM or SUN, and you are now lvl 18 try to buy or find a lvl 18 same slot item with blues or + .. that's the way all people do here.