You need to be more specific about what's wrong. Two of the most common failures are:
1. Foam edge of speaker has rotted away.
2. Voice coil is damaged.
Re-foaming takes some practice, but if I can do it, so can you.
Re-coning takes a lot of knowledge and specialized materials and tools. It should be done by an experienced person.
A lot of times it's cheaper and better to just buy a replacement speaker and put it in the cabinet.