I read about this somewhere:

"the center dome on MOST speakers (like on your's don't worry) are just covers for the center area, so there shouldnt be any real change in sound


you can also take a piece of gaffer tape , stick it to the dent part and gently pull it out."

or: vacuum-cleaner but

"see if you can use your mouth before trying a vacuum. It can be a bit of a delicate process."

gaffa tape seems to win