my Center speaker ( JBL ) is 10 years old, the Adhesives foam is dry and started to crumble.
I have ordered from eBay a kit for replacing it , took me 30 min and it`s done.
1.measure the inner / external diameter of the speaker and order a Repair Kit from eBay
2.remove old crumble foam with some utility knife ( Stanley knife ) , do it slowly . be careful not to destroy the inner part of the speaker cone.
3. put the glue that comes with the kit ,slowly put the Adhesives foam on top and gently tap on it to make it stick to each other .
4. leave it to dry. and it`s done!