Use a moon stone on it.
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Jigglypuff will evolve into Wigglytuff with a Moon Stone.
No matter what game it is, you need a moon stone to evolve Jigglypuff.
You can't evolve Yanma in Leafgreen. This is because Leafgreen was made before Yanmega was made. So basically in Leafgreen, Yanma is a Pokemon that can't evolve.
Moon Stone.
You can't on leafgreen.