An Upgrade is a replacement or enhancement of an existing program or device. They are loaded into a system by the User. Some can be very large, others are smaller, they can contain weapons, advanced technology, or personal enhancements.
During an upgrade the systems sky will turn bright yellow with bursts of electricity. The System Voice will announce, "Attention, Incoming File," several times and a hole will open in the sky above the Principal Office. The buildings Sub-Sphere will be above the building, ready to receive the upgrade. Soon the upgrade will quickly fly down through the hole with a trail of white dust behind it, landing with a loud bang on the top floor of the Sub-Sphere. Then the cities leader will order the Sup-Sphere into the Core where the upgrade will be installed. The finally process before instalation is to put up the Principal Office defenses, and power down the city in preparation for it's upgrade.
Mainframe received a small upgrade during Bob's second year in the small system. He was surprised at the small size of the container the upgrade arrived in. Dot Matrix told him that they're not in the Super Computer, and Phong explained that the User sends what will make their world a little better, usually.
Megabyte and Hexadecimal created a fake upgrade and were able to break into the Principal Office by hiding Megabyte in the upgrade canister. Bob and Dot were able to defeat him, and the true upgrade began when he was thrown from the Principal Office to Silicon Tor. (Infected)