You level him up on a high friendship level, I suggest the "sooth Bell" found in the Pokemon Mansoin second room to the left.
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It evolves via happiness
It evovles with max happiness
You need to get it's friendship to full.
Munchlax evolves into Snorlax via max happiness in Pok
To evolve the Munchlax you just have to level it up when its holding the leftovers.