Performing such a repair will require two things.
First, the type of heavy duty elastic in the proper width, which is available only from a few sources and generally comes in rolls of 100 feet or so, and is rather expensive.
Second, the holster will have to be completely disassembled, the new elastic installed, then the holster will have to be completely reassembled. If the stitching is done on a machine it will be unlikely that every stitch-line and hole will be 100% aligned. If the stitching is done by hand the lines and holes can be matched up, but the work will take much longer.
So, the cost of the elastic will probably be more than the price of a new holster and the work involved will easily be two or three times as much as making a new holster from scratch.
You may find someone willing to take on the job, but they will probably be sorry they did it.
Best regards.