Canon imageRunner 400 Reference Manual page 222

Finisher-C1/Saddle Finisher-C2
7–28
4
Pull out the empty staple cartridge holding it by its
left and right sides.
5
Insert the new staple cartridge.
NOTICE
Replace both the inner and outer staple cartridges at
the same time.
6
Pull forward the stapler unit of the saddle
stitcher unit, then return it to its original
position.
7
Push back the saddle stitcher unit,
then close the front cover (lower).