How do I buy or upgrade weapons in survival mode while playing solo?

U go to the gun shaped icon to upgrade you're gun

Well, you get points for killing enemies, and these points can be used to buy upgrades and such if you are at a high enough level. Look for the gun symbol, it will lead you to a laptop that you can buy different guns on, and upgrade guns you already have.

In this Picture there is an Air Support Symbol, a Grenade Symbol, and a Gun Symbol. Look for the gun symbol on the level your playing to find the upgrade laptop. Click on the Image to enlarge it.

You get points for killing enemies and then you go over to the crate that has a gun logo and buy something

If you want the to change or upgrade your gun in the map Dome you go inside the dome and follow the gun icon. If you have an Xbox 360 hold down the X button and if you have a PS3 then you hold down the square button. When you have found the gun type you want you click the A button on that type. If the gun you want and the price is red then you cannot get that gun. To upgrade a gun click on that gun and then click upgrade. Then it will give you a list of options that you can pick from Ex. Red Dot Sight. After you have made your selection click B or circle and then press B or circle then you are ready for your next round. The gun icon will not come up on the first level but after the first level you can change or upgrade your gun.

You have to kill people and do the challenges on the side for much better rewards. You must level up to buy more weapons and attachments. After you do the first wave, the weapons crate will be unlocked, just go to it and you'll access it.