I posted this a while back on Watch Repair. I had the same problem a year ago. This is the method:
1) remove movt
2) push out watch glass - tip; wear gloves, push out with thumb(s)
*be very careful as you may scratch the slide rule or break the glass!*
3) lift out the slide rule insert to reveal the bezel screws!
4) tighten the 3 little screws, but not too tight or the bezel wont spin!
5) refit and clean glass etc. Refitting the glass is either a Pro job, or DIY which has an element of risk. I carefully lined up the glass in place, then pressed down in 3 positions (ie. 120 degree spacing) until the glass went in. The I pressed the whole case (glass facing out, pressure on glass edges) onto a wooden wardrobe (or door frame!), something immobile, to seat it fully. Always keep the pressure on the edge of the glass, not the centre.
Hope that helps!
ps. don't be tempted to remove the steel bezel (unless its really dirty) as there's an o-ring underneath which is VERY difficult to put back in place. I damaged mine and had to get rid of it.