To change your bike inner tube you have to remove the wheel from the bike, take off one side of the rim be careful not to pierce the tube in between or it will prove difficult to fix later on, and remove all air from the tire so it won’t stick out when you pull the old tube out. This is very important because if your tire sticks out too far, it will no longer fit back on the rim and you’ll have to take the other side off as well.
To take all of the air out of the tire, use a refill thingy (can’t think of what else to call it) that comes with the inner tube and has a hole you can put over your bike’s valve stem. When you push down on it, it sucks all of the air out and seals itself off automatically. Repeat this process 2 or 3 times until almost no more air is coming out, then quickly shove your finger over the hole to make sure no more air comes out. If you don’t do this, your tire will stick out too far and you’ll have to take one side off the rim as well. So now you should be able to pull your bike inner tube out without much resistance.
After removing it, find a patch and put oil on it (to help the glue stick better), and then apply it to the area where your tube was (on the outside of the tire).
It’s time for a new tube. You can either get one that matches your old size or get one that fits just right. If you want the latter, just squeeze out some air from your old tube so it’s flat and hold it against a new tube. You can then cut off the end of the new one that corresponds with your old tube’s end and the two should be exactly the same size. Make sure you measure it all along its length as well.
Once you’re done, put the tire back on, making sure to screw in both sides as tightly as possible. If the tire isn’t on all of the ways, it will stick out too far when you inflate the new tube and the whole process will have to be repeated. You can then put air back in by using your pump until it feels nice and firm again.