Spare Jeans button - 2Nos; 1" cloth patch pieces, Hammer, plastic jean-button replacement tool (optional)
Remove the old button
Make small snips with seam ripper to take the button out
Reinforce the hole with extra fabric underneath Use hammer to attach the two pieces of the button through the hole. Use a buffer to protect the button
If you do not have a new jeans button make one easily with fabric and any other shank button