Here's a step by step from memory. The memory is of Signmaker, Sprint and SuperSprint plotters so there may be some minor differences you will need to interpret with the HS-15. You will need a Phillips screwdriver and a set of Allen wrenches.:
1. Unplug the plotter from the electrical outlet.
2. Remove the Phillips screws that hold on the two end covers and remove the end covers. This may vary with the keypad on the front of the HS-15 and you may only want to remove the rear end cover.
3. Locate the motor that drives the X-axis drum. It is held in place by four Phillips screws. Make note of its approximate position, remove the four screws and remove the motor.
4. You are now looking at the gear that is driven by the motor. It is attached to a stainless steel drum support shaft that runs through the X-axis drum. It is held in place by a Allen screw collar clamp. The drum support shaft is held in place by a second collar clamp located just the other side of the chassis wall on the drum itself. Later, when you install the new drum you will want to adjust the side to side slop. So at this time, spin the drum an note the relative freedom with which it spins and how little if any side to side slop there is if you try to wiggle the drum from side to side.
DO NOT loosen the collar clamp on the gear, but DO loosen the Allen screw on the collar clamp on the drum.
5. Place one hand on the X-axis drum to prevent it from moving and with the other hand turn the drum drive gear while also pulling it straight out of the drum. The gear and the support shaft will come completely out of the plotter. Left behind are the drum, the collar clamp and one or two washers that ride on the shaft between the drum and the motor end roller bearing. Make note of the order of these washers. Also make note of which end of the drum has the sliding sprocket.
Optional step 5A. Inspect the drum support shaft for any significant wear at the two points that in was in contact with the two roller bearings. If wear is visible, loosen the collar clamp on the drive gear, remove the gear and clamp, reinstall on the other end of the shaft and tighten the collar clamp. Because the contact points are different, this is the next best thing to replacing the drum support shaft.
6. Remove the drum and washers from the plotter. Place the drum collar clamp on the new drum in the same orientation as on the old drum. Insert the drum support shaft slightly through the bearing on the motor end and place the washers on it in the same order as when removed.
7. Insert the new drum into the cavity and work the support shaft all the way through it and the roller bearing on the other end.
8. With your fingers on the sprocket and your thumb on the drum drive gear, gently but firmly squeeze together to remove any side to side slop and tighten the drum collar clamp. Check the drum for any side to side wiggle (there should be none) and that it spins freely as experienced in step 4.
9. Reinstall the motor. Note this is a slotted adjustment for how tightly the motor gear meshes with the drum drive gear. As with step 8, a gentle but firm squeeze is what is needed.
10. Plug in the plotter and slew the plotter left and right listening and looking for anything that doesn't sound or look right. If satisfied, unplug the plotter, reinstall the end covers, plug it back in and you're good to go.