You can upgrade your airsoft gun, just depends on what type you have , and what you want to do to it. You can upgrade the barrel to get a higher fps on it. you can upgrade the spring on electric guns but you will need added parts to go with this upgrade.
Please refer to this link and it will help you with parts needed
http://www.airsplat.com/Categories/AC-UP.htm
And I included a maintenance link for you aswell
http://www.airsplat.com/Airsoft-Gun-Maintenance-Guide.htm
You can find all items and info at www.airsplat.com
Some common upgrades for AEGs include:
Tight bore barrels
High torque and/or high speed motors
piston and piston heads
springs and spring guides
air seal nozzles
High performance gear sets
re shimming gears
Most of these mods require disassembling the gear box, but if you buy compatible parts, they should be relatively easy to install, easiest being either the barrel or spring/ spring guide (depending on the gun), and hardest/most time consuming being re-shimming gears. Any of these mods alone, or any combination of these will have a noticeable impact on the performance of your gun (the only exeption being the air seal nozzle, which will help tremendously if there are air leak problems, but will do pretty much nothing if there aren't)
Not AFAIK
Double Eagle guns are cheap Chinese toys. They cannot be upgraded. You should buy Classic Army or Marui airsoft gun, they are easy to upgrade.
An Airsoft gun
A pump airsoft gun is a gun that has to be pumped every time before shooting. Ex. Airsoft shotgun
No
No
It is an airsoft gun based of the PPSH-41 gun
Yes you can
You never need a license to have an Airsoft gun.
No.
No
NO