To access the light bulb, use the button-hole foot to lift up the insert on the arm cover. Remove the cover. Hold the edge guide slightly tilted and push it into the opening in the head of the sewing machine. Press down on the lamp housing to hold it still. Push up on the bulb and turn it counter-clockwise. Once the bulb is loose, pull it down and out of the lamp housing. To replace the bulb, continue to hold the lamp housing. Push the new bulb into the housing and rotate it clockwise. Release the lamp housing and then use the button-hole foot to replace the cover.
Pfaff has the manuals for most of their machines here: http://www.pfaff.com/en-US/Support-and-Updates/Support
. The Hobbymatic 955 support page is http://www.pfaff.com/en-US/Support/3529
. The direct link to the manual is http://www.pfaff.com/SiteMedia/PFAFF/Products/Machines/Support-manuals/Pfaff_935_955_HobbyMATIC-manual-EN-FR-DE-NL.PDF
. See page 51 (page 59 of the PDF) for the directions to change the light bulb, These pictures may help you see how to get to the bulb. If you need a replacement button-hole foot, see http://www.sewingpartsonline.com/buttonhole-foot-low-shank-new-1932l.aspx .
I hope this helps.