Usually is there a fee to modify hotel reservations?

I am wanting to change reservations from 3 rooms for 6 people to 2 rooms for 6 people. It is a little cheaper this way. How much does it usually cost to modify hotel reservations? I did pay in advance... Then approximately how much does it cost to change reservations within 24 hours of initial stay? travelforless4utravelforless4u

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