Formal:
* KOO-pon (with a long "oo" sound)
Informal:
* KOO-pon (with a long "oo" sound)
* KYOO-pon (with a short "oo" sound)
Both are acceptable, and the pronunciation you choose will likely depend on your regional accent and personal preference.
It's important to note that some people may pronounce it with a different stress pattern, such as "COO-pon," but this is not considered the standard pronunciation.