To remove the groundglass back, you push the two ridged silver buttons on the top and bottom of the groundglass frame (oriented horizontally) and slide off the whole groundglass assembly from the back. Then you should be looking at the surface in the back that a normal 4x5" filmholder would rest against. At the top and bottom of the back there are black slides that hold rollfilm holders and other Graflok-type accessories in place. If the rollfilm holder is too small to fit this back, then you have a 6x9 back designed for a 2x3" camera, and it won't fit a 4x5" camera, unless it has a 2x3" reducing back, which is an unusual accessory (though I happen to have one for Linhof).