Question

how does rate of evaporation changes with increase in humidity

Answer 1

Evaporation is the transfer of water from the liquid phase to the vapor phase. 

If the air is fully saturated with water, then no net amount of water will evaporate from the liquid phase so increasing humidity decreases the evaporation of water.

Answer 2

The rate of evaporation gets slower with increased amount of humidity in the air as there are already lots of water present in the atmosphere or air so the air cannot hold more water vapour that is the reason why the rate of evaporation gets slower