dent on macbook air 13''

You will need to get a quote from an Apple Store or an Apple Authorized Service Provider. Usually if the display issue is caused by a dent, then it will be considered Accidental Damage and will not be covered by the warranty. If you have AppleCare+ with any damage incidents left, then the repair cost will be a partially subsidized. See the AppleCare+ information for rates for replacing a damaged Display.

https://www.apple.com/support/products/mac/

If the dent on the Display is causing visible issues on the screen then you will need to have the Display Assembly replaced by Apple or an Apple Authorized Service Provider.