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.
Dimple in speaker driver cone
I have a pair of Thiel CS 1.6s that have a quarter size dimple in one the driver cones of one speaker that a friends curious child created when he pushed his finger into my speaker (damn him). Can that dimple be removed by moi? Or do I need to have it repaired or replaced by the manufacturer?