The one that screws into the larger flat.
Thru spindle too short on door handle.
If that does not work try and use a thin scraper or similar to disengage the latch from the gap between door and frame.
Once door is open remove old latch and replace.
The other end of the spindle and opposite knob is threaded so that this knob can screw on to the spindle and subsequently adjust for door thickness.
One knob is fixed permanently to the spindle by means of a grub screw through the neck of the knob.
Take old latch with you as there are numerous different sizes.
Hold the knob in one hand and insert the other end of the spindle into the square port inside the door lock.
Remove the knob and rosette from the other side of.
I do this most days easiest way to do it is to insert follower bar thats the technical term for the spindle into the door handle and through the latch in the door ensure door handle is.
It s nice being able to get through your door without too much trouble.
It seems the original had broken so it actually was too short to fit where it belonged.
Mount the handle near the end of the the spindle on the spindle opposite the one that contains the larger flat ie.
Remove the rosette off of the spindle.
Unscrew one handle and remove the spindle.