How do I fix/replace tyre pressure sensors?

First thing to do is figure out which one went bad. The issue here is they are inside the tire and unless you know how to dismount the tire, swap it and have the equipment to program the sensor in this isn't a job me nor you can do. Some cars tell you with 0 PSI on the cluster but I have seen it before where you need to use a scanner with TPMS support to figure it out. It is kind of pricey but the Innova 5610 can scan it as well as the 5510 - the 5610 is just bidirectional. You can also try to reset it and put 15-20 miles on the car and see as a hail Mary.

The good thing is the part is cheap - ~$50. The cost is in dismounting the tire and programming :-(. If your tires are coming up soon I would ignore it and do it then and in the meantime see which ones are affected. Generally speaking what I usually suggest is do the bad side in pairs - if the passenger front right is bad do the driver and passenger on the front. They tend to follow and if you lose one the other one generally doesn't last much longer. It will cost you a bit more but it will save you labor.