Slide-operated coin mechanisms are used for a number of coin based services, from newspaper racks to coin-operated laundry machines. These devices have vertical slots into which coins are put (typically quarters) that are set in width to match the width of specific coins; some can also take tokens that are shaped like coins. Resetting a slide-operated coin mechanism needs to be done when a price is changed on a device, or, rarely, when it's jammed.
Unlock the coin box and remove any coinage out of it. If clearing a jammed device, you'll usually see the bent coin or token causing the jam at this point.
Work the slide back and forth with the box open and look for places where it might be sticking -- applying some machine oil can fix most problems.
Remove the cover plate from the top of the coin box -- this plate will be towards the front, and has several grooves for coins (typically 6 in a laundry machine), each with a plate that can be screwed into place. Different plates (which are provided by the manufacturer) have different widths to accept different sizes of coins. Unscrew the plates and replace them with the ones you need to match the price of the service you want.
Double check the width of the "pincher" at the back of the plate to make sure the proper coin can both go through the plate, and pass through the pincher without being too loose. (Loose coins won't register as payments; this is how the coin slide mechanism prevents people from using nickels instead of quarters to pay for laundry).