Replacing keys is a matter of (summed up)
1. opening up the keyboard - in some cases, the outer case needs to be taken off. (In other cases, though, the individual keys can be remoted, exposing the keyswitches below them, without removal of the outer case at all. I'm not sure which type yours is yet...)
2. taking off the key (that your fingers touch when playing) from the whole keyboard.
3. assessing whether the keyboard is damaged inside, or whether the keyswitch (a sort of small button that the keys your fingers touch press on) is in need of replacing, or only the key parts above the keyswitch.
4. taking appropriate action.
But there is a step to try before those steps: taking compressed air and spraying into the cracks between the keys that aren't sounding and the adjacent keys. An air compressor of the sort used to fill tires can be used for this (provided it is used in a clean area and does not suck up dust and introduce it into the keyboard.)
Press down keys adjacent to the ones that don't sound, and with the compressor, spray air into the cracks between the depressed keys and the not-sounding key.
That sort of forced-air cleaning can revive a clogged keyswitch that has gotten dust or crumbs into it. (If it revives your keyboard, then a caution against ever eating anywhere near it, would be appropriate.)
Let me know what you think of this. Thanks.