Hi and welcome to JustAnswer,

Thank you for the question and your patience. Should you still need assistance for information purposes only...

Initially, it would be best to have all the surrounds replaced at the same time; this would be predictive/preventive maintenance. And it would allow you equal basis when comparing performance with the existing fully working speakers.
Offhand, considerations for repair:

• • select a source of the surround near you;
• • if at all possible and preferably, select a source that could also do the job of actual edge/surround replacement;
• • the above has the advantage of immediate availability of a surround replacement if for some reason the job was not perfect;
• • changing/replacing the surround is not that difficult. The critical part is maintaining the position of the voice coil so as to retain its clearance/space to/from the magnet;
• • surround replacement calls for the use of adhesive; a rather messy job even with jigs and implements that may even be not readily available with onetime DIYers;
• • patience, techniques, hands-on-experience required. Perhaps asking a professional (source near you) to do the repairs would be more to your advantage.