The easiest fix is to take the item to an upholstery or canvas shop to have the snap replaced. It’s typically a quick, low-cost repair. If you’re local, this can also be done at the Mothership.
For a DIY repair, remove the damaged snap and install a new one. This Burley kit is a good low-cost option. Another alternative is a kit from Amazon (one of many options). These often include more hardware than needed but may come with pliers-style crimpers, which many people find easier than the hammer method used with the Burley kit.
While snap sizes vary, most use common sizes, and the kits linked above are typically the correct fit.