Drill bits inevitably become dull. It is a way of life. So here is a way to bring them back into life. This is more for field work than for precision machining. If you are careful you can do a nice job and have a precise sharp drill at the end but it can take quite a bit of practice to get there.
To begin with we have to remember that we are only trying to get a tiny part of the bit to be sharp. So this is about removing the least amount of material that you can to make the bit sharp.
We are going to touch just the back side of that yellow section of the bit. Also look at that image and see how both halves are the same size. As you are doing the grinding make sure that you do exactly the same thing to both sides. If you spend too long on one side of the bit you will move the center of the bit over and then the bit will drill with a wobble that gives you a much larger hole.
So the easiest and fastest tool to use is a belt grinder but all kinds of tools that remove material will work. From grinding wheels to angle grinders as long as you get the location right you can point up a drill bit.
When sharpening a bit the important part is the alignment of the cutting edge of the bit to the correct part of the grinder. The most important part is to keep the cutting edge of the drill perpendicular to the direction of travel of your grinder.
This means holding the bit at an angle to the surface. In order to sharpen the bit you also have to hold it at an angle down from the grinder.
Once you have the angles right you need to touch it to the grinding wheel and then flip the bit That way you keep taking off material at the same rate from both sides. It should not take a lot to get an edge back on the bit. If you take too much off then you will make the very tip too wide and then you have to learn how to split point a bit. Which takes quite a lot more experience.
If you are using a hand grinder to point a bit up then there is an additional tip that helps. Since you are probably holding both of them and they can both move around you need a rest to keep the bit from moving around too much. So start the bit onto the grinder above the cutting edge and then raise the bit up so that the cutting edge hits the grinder. That way you can work your way into the edge in a controlled way instead of tapping your way there and taking off parts of the bit you did not want to.