The kit is used to upgrade an event building.
You may know this already, but let me clarify this statement in case you don't. A One-Up kit is not the same as an Event building Upgrade kit. A One-Up kit will move an existing event building up one age, not upgrade an Event building to the next level.
For instance if there is an Event building that when fully upgraded is 10 levels, but during the event you were only able to build it up to level 8, a One-Up kit will not bring it to level 9.
What it will do is if you built your Level 8 Event building in Iron Age (IA) and your city is now in High Middle Ages (HMA), a One-Up kit will advance your Level 8 building up one age to produce resources from Early Middle Ages (EMA).
Using a building that produces goods for example, having been built in IA, it will continue to produce IA goods regardless of your current age. If you use a One-Up kit on it, it will no longer be a Level 8 event building producing IA goods, it will then be a Level 8 building producing EMA goods. To have it produce HMA goods, two ages up from IA, it will require a second One-Up kit to bring it from EMA to HMA.
A renovation kit on the other hand will bring an Event building from IA to HMA with a single kit. That's the difference between the two kits, One-Up advances the building one age, a Reno Kit will advance the building to your current age regardless of the number of ages between the age you built the building and your current age.
I Hope that helps.
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