Glass Bezel crack on Macbook air

I would cut a piece of black electrical tape to cover it to contain the break....just make sure the tape extends past the break...probably to about the gap between the two brightness keys.

Unfortunately Apple doesn't replace just the broken bezel, but the entire Display Assembly which is very expensive. Even if you have AppleCare+ coverage to help defray the repair costs for accidental damage, the repair cost would still be $99US (full price can easily be $400+US depending on the model). You can check Apple's repair prices using the following Apple link:

Mac Repair and Service - Apple Support

Technically a skilled tech can remove the broken Bezel and replace it with a good one, but very few techs have that skill. Even a skilled tech can accidentally damage one of the many thin fragile flex cables located beneath the Bezel. I've personally replaced Bezels on the Intel Macs, but I also have a lot of previous experience that helped me to perform those repairs (it takes practice).

FYI, those Bezels are very fragile and can easily crack if any pressure is applied to them since there is mostly hollow space beneath the Bezel (the display's many flex cables don't provide much support). Even a note card caught between the display & keyboard can cause the Bezel to crack. And like @bhanu5567 mentions, an impact to the laptop can also cause the Bezel to crack (not always, but sometimes).

The bezel damage will happen mostly when Mac is drop from certain heights or collided with some hard maetals.

This comes under physical damage which cannot be covered under warranty.

If you have an care plus for the device , you may replace the display for free or less cost .