The spool holder is a 3D printed part that holds the filament spool and the feed tube, keeping it stable and secure to the system. Be sure to print a spare spool holder prior to replacement.
Before your spool holder breaks, 3D print a replacement to have on hand. If you didn't preemptively print a replacement you can still print without a spool holder. Just find some way of holding the spool so that it can rotate as the filament gets pulled off.
Go to "Maintenance" and tell the printer to withdraw the plastic filament from the extrusion assembly. Finish removing the filament by pulling upwards.
Remove the plastic filament tube from the spool.
Remove the spool from the spool holder by lifting it up and taking it off.
Slide the spool holder upwards, and then pull outwards.
Place the new spool holder by pushing it inwards and sliding it down until it locks.
Repeat the steps in reverse chronological order to reassemble the printer.