There are a couple of different ways that you can remove the angle grinder disc. Two flattened wrenches come with many of these tools. The one that looks like a two-pronged fork can be inserted in the drive bolt between the disc and the angle-grinder motor. It should be turned in the opposite direction of the disc direction arrow while using the other wrench to stop it from turning. There may also be a button which holds the disc in place.
Another option is to use improvised spanners. First, you need vice grips to secure the bolt holding the disc to the drive bolt. Using the grips to stop the disc spinning, you can then insert a screwdriver between the disc and the angle-grinder motor. You need to pull it in the converse direction to the arrow on the disc. If you can’t loosen it yourself, you can use a hammer to encourage the screwdriver to turn.
Your final option is to not use any tools at all. First, you should strike the disc in the reverse direction of the arrow on it, loosening the nut enough to take it out with your fingers. If you are unable to do this, you can break the disc off in pieces to get the best access to the attachment nut and bolt.