How do I clean my MacBook Air keyboard for softer keys?

Here is an Apple article with instructions for cleaning the Keyboard of crumbs & debris, just make sure to keep the can of compressed air completely upright or you may damage the laptop with cold wet propellant:

How to clean the keyboard of your MacBook or MacBook Pro - Apple Support

More than likely you are dealing with accidental liquid spill (or splash), or from crumbs filled with moisture. Trying to remove key caps very often results in the fragile key cap and/or the scissor mechanism becoming damaged during key cap removal or reinstallation. How each key cap is removed varies from keyboard to keyboard...even similar keys on the same keyboard may detach differently.....Apple will often source two different manufacturers for keyboards which results in having two completely different physical key caps & scissors. Then the various regional keyboard differences add even more variations. This means no single set of instructions you find online will provide you with the exact method to remove any key caps on any specific keyboard.

If only a few keys are spongy, then perhaps an Apple Authorized Service Provider may be willing to remove those specific key caps & scissors to clean them of any liquid residue. Unfortunately it is unlikely Apple themselves will do so, but you can always ask.

In my own personal experience supporting my organization's Macs, usually the only way to clean them up is by removing the key cap & scissor mechanism. Even though I have done this a lot, once in a while I do end up damaging the key cap or scissor mechanism, but I also have access to a lot of scrapped keyboards to use for replacement key caps & scissor mechanisms so I usually have something available to fix anything that may be accidentally damaged.