Dimple in speaker driver cone

You can leave it alone. It will not alter the sound but only annoy you visually.

If it is in the center of the speaker it is in the dust cap of the magnet. If this is the case it has no effect on the sound and can be ignored or repaired by you. I had one recently which I repaired by judiciously pushing on other side of cap, another method I have heard of but not used involves using a very small pin with a bent end to pull it back in place. If it is really a dimple in the cone itself then professional help is required.

You can also try adhering a piece of tape to the center of the dimple and pulling it out or if you are daring a small vacuum cleaner.