OK. You'll need a very small flat screwdriver, a file, a pin punch, drill, and a vise with anvil.
At the ends of the knife, inside the liners, you will see a small tab that is folded over the liners, from the bolsters. Use your screwdriver to bend these out just enough to clear the liners on one end only! Lift the handle until the tab clears and slide it rearward, so the other tab slips out. Remove the handles, and you will see 3 pins. The center and rear pins hold the backspring in place on a single blade knife. On a dual blade, only the center pin holds the backspring.
File the head off the rivet(s), and punch them out with the pin punch.
Remove the backspring and examine it for wear or cracks. If no cracks, gently bend the backspring to increase tension. Use a small common nail to replace the pins. If you can't find the exact size, chuck a larger nail in your drill and use it like a lathe to turn the nail down to exact size with the file.
Put the backspring in place and install the rear pin. The backspring will stick up at the pin hole. Place the knife in the vise, and gently squeeze the backspring down until you can insert the pin.
Try the knife. If it works satisfactorily, cut the pins off, leaving about 1/16" sticking out on both sides. File the ends of the pins flat, place on your vise or anvil, and GENTLY peen the pins with a very small hammer. Use light taps! Alternate sides until all the slack is taken up.
Slide the unbent handle tabs back into the rear of the knife, and use your screwdriver to fold the tabs back over flush.
Note: the tabs are fragile, and you probably can only bend them once!