Upgrade is a Galvanic Mechomorph (mecho- meaning machine and -morph meaning shape) of the moon Galvan B, which orbits Galvan Prime.
Upgrade is the only alien form in which Ben's voice does not change, aside from the more computerized tone behind it. The reason for this is revealed in the enhanced version of "Game Over"; Galvanic Mechomorphs cannot bond with living organisms, so Ben's transformation into Upgrade is incomplete. Upgrade is composed of nanotechnology, allowing him to reshape his body at will. This makes restraining or harming him difficult, as he can "melt" to avoid either and will quickly regenerate from any damage taken. Upgrade can also merge with other technology, earning his name by upgrading it far beyond its original design as long as he is merged with it. The size of the device is inconsequential. While merged with a device, Upgrade controls it as he would his own body. Even without a machine at hand, Upgrade can reconfigure his form around his eye to fire a positronic plasma beam from it, usually when angry, although Upgrade has rarely used it. Upgrade is also capable of forming simple constructs from plasma, such as spikes. Upgrade's weakness is his specialization: being technology-oriented, Upgrade cannot possess biological creatures, and sentient or more powerful robots are harder for him to control, he is also weak against electricity. Upgrade's liquid-metal composition can also work against him, namely where electrical attacks are concerned. Furthermore, any sort of metal corrosive can upset Upgrade's delicate internal workings and wreak havoc on his systems. 
Ben first transformed into Upgrade in "Permanent Retirement" to stop some criminals who were robbing an ATM