How do you replace steering coulm on Saab 900?

Since you didn't specify, I will presume you mean the Classic 900 (1979-93).

Remove the steering wheel, remove the knee pad under the dash. Remove the turn signal switch being VERY careful to note which wires go where otherwise you'll have a big mess on your hands. Remove the wiper/washer switch.

There are four bolts that hold the collapsible cage for the steering column in place, plus the bolt at the bottom that connects everything to the steering rack.

Be very careful in re-installing everything as the alignment on the cross member is critical. If you get it wrong, the replacement cage will break when installing.