Thanks for asking this question on JustAnswer.. Since I don't know what machine you have I will give you the most common answer that should work with most dryers. First unplug the dryer. If the lint filter is on the top and not inside the door you will need to pull it out of it's slot and remove the 2 screws that hold the filter tube to the lid.
The top of the dryer has a seam around it about an inch down. In that seam, or crack. If you take a flat screw driver and place it in the crack on the front and near the left or right side and pry it apart just enough to see in the crack you will see a metal clip a few inches from the side and on both the left and the right. You can move the screw driver to right in front of that clip and then push in on it. (like you are stabbing the crack) As you push in you will release the clip and the top with come loose from the front face of the dryer. You must do that to both of the clips on the front of the dryer. If you get them both released you can slowly lift the top as it hinges on the back of the dryer like a door. Slowly I say because there is a wire that sometimes is attached to the lid that goes up to the front of the dryer from the back and you must slide the clip off the lid to free it up. With the lid up and against a wall or some thing to hold it up, tape etc. You can look down into the top of the dryer and see the barrel and a thin belt wrapped around it and going some place under it. That is your belt but you can't remove it yet. You will have to remove the face of the dryer. The wire that runs to the front of the dryer goes to a switch that is closed by the dryer door closing. This wire can be unplugged from the switch and unclipped from the frame down the front.
The front panel is like the top and has a seam around it, going down both sides. If you look inside where the front is attached to the sides you will see screws that hold it in place. They are on the inside and they screw toward the front of the dryer. Remove these screws. On the bottom front on some dryers is a panel, like a kick panel and it has screws in it that must also be removed. Some just have the top screws and you can hinge the front panel away from the barrel and the sides for a few inches. The bottom is connected with 2 holes that slide over 2 posts or pins. If you lift on the face it will slide out of the pins and become loose from the bottom. There are a few ways that the barrel is supported, one is the face plate has a felt rest for it to set on, others there is a bracket that comes up from the bottom or the dryer floor. But as soon as you can see under the barrel you will see the motor and notice the way the belt is attached, with a spring or other means of tension. Look at it and record how it is assembled (digital cameras are good memory tools) Release the tension from the belt and remove it from the motor it will then me loose on the barrel. Move it to the front of the barrel and slide it off. You might have to lift the barrel up a bit off of felt or wheels to get the belt through and remove it. Then you just place the new belt back the same way and reassemble the face, replace any wires to the brackets and where you had to unplug them. Put the screws back in the face from behind. Close the top back down, lining up the clips with the holes in the lid and close it snuggly. Striking right over the clips with a rubber mallet is sometimes required to get them to lock in place. Line up the holes for the filter if it was on top and put the screws back in, replace filter. Put the bottom face plate back on and you should be back in business.
Hope this described what your dryer looks like, good luck. Any questions just ask.
If this worked for you please accept my answer. Thanks