Hi there's a pull string in front of the bottom half of the lower seat. One for each side. Pull the string and remove the lower seat out of the car.
Next look around for a loose bolt. If there's no bolt, then look near the bottom of the upper seat there should be two metal like U hooks attached to your cars body almost near the seat belt tie down. This is the ancor point for the upper seat.
You will have to get a new bolt if your missing one or both. (Theres two bolts altogether) that ancor the upper seat to your car.
To put your seat back in you have to lift the whole seat up, and towards your back window, then back down. Theres hooks to hang it on the back. Also check the back of the seat to make sure your bars to attach to hooks are good. If your having a hard time then you might have to re-adjust the three piece seat to fit propely. After you have it hanging screw the bolts back in at the bottom U ancor. And then you can put back your lower seat. Make sure the little claw like things are open to all of that just snaps back in.
Ted 06 M6