Upjet is a term describing the action of changing the size of the main jet in a moped's carburetor to a larger one. Upjetting can be done either by purchasing a new main jet from moped parts suppliers or by drilling/boring out the existing main jet. For Bing and Dellorto jets, the smaller the number (size), the smaller the hole. Upjetting will require a jet with a larger number (size).
When adding performance parts to your moped, it is very important to maintain the proper fuel/air ratio. Any part that increases air flow through your engine (and most performance parts will do this in one form or another) will almost always need to be accompanied by a corresponding increase in fuel. Failure to increase fuel flow can result in engine seizure. You increase the fuel flow by replacing your main jet with a larger main jet.
How do I upjet?
Before you can order new jets, you have to know what size jet is already in your carburetor. Remove the float bowl from the carburetor (depending on the moped, you will likely have to remove or disassemble other parts to be able to get the float bowl off). Looking at the bottom of the carb, there should be a small brass-colored screw, usually in the center of the carb. This is the jet, and it will have a number stamped on it. That number is the size of the jet. When your new, larger jets arrive, you can use a screwdriver to take out the old jet, and then screw in the new jet. After reassembling and reattaching the carb, it is a good idea to check for air leaks and run a plug chop to verify that the new jet is the correct size.
Videos that might help:
What size jet do I need?
Correct jetting is dependent on many factors. Air leaks, differences in parts or part installation, temperature, altitude, engine, model, and other variables can all alter your jetting needs. This is why it is very difficult for someone on an internet forum to pinpoint the exact jet you will need. It is recommended that you buy a range of jets and determine for yourself which size works best for your situation. Less involved performance upgrades, like a high-flow air filter or an expansion chamber exhaust, will generally require a smaller increase in jet size (maybe 1 or 2 sizes). More involved performance upgrades, like a kit or larger carburetor, can require a larger increase in jet size. More information on performance upgrades is available in the performance tuning category, and you can explore setups and jetting in the spreadsheet.
Where do I buy jets?
Treats HQ and 1977 Mopeds are two of the larger and more commonly mentioned parts suppliers that carry jets. However, many of the other moped shops also carry jets (and plenty of other goodies, so check them out).