You should have gotten 5 o-rings in 3 sizes.
The large o-ring goes on the lid, just peel off the old one and replace with the new, it should seal the lid to the keg.
The next middle size is the post o-rings (should be 2). They fit into the groove on the liquid and gas posts on the keg. The old ones can be kinda hard to get out because they are tight... you can pry em out with a very small screwdriver or if needed you could even cut them. The new ones are not hard to put back on.
The smallest o-rings are the dip tube rings. You have to remove the posts to replace them. The posts (gas and liquid) are threaded onto the keg. With a socket you should be able to remove the posts (at first glance they look the same, THEY ARE DIFFERENT, keep straight where they go back on the keg). Under the posts, you'll see the top of the dip tubes. Once the posts are removed, you can remove the dip tubes by pulling up or pushing from inside the keg (make sure you keep the posts and the tubes together and attached to the correct side of the keg). The short tube is the gas tube, the long one is the liquid out tube.
The dip tube o-rings then just slide off the tubes and the new ones slide right back on.
I suggest you get and use "keg lube" to lubricate the new o-rings to help keep them soft and seal up.
The post o-rings are the ones that take the most abuse, but I would think you should get at least half a dozen uses or more (I've never had one that I knew had to be replaced).
