I like randomnumbers solution above but this might help more fully and explain how to do randomnumbers fix if you can't figure how to separate the nozzle from the collar
The spray top has two components, the combined nozzle and push button assembly, and the collar. The nozzle turns around inside the collar and might be stiff. You have to grip the collar tight to stop it turning with the nozzle head.
On the collar are two faint marks, O and 1
On the nozzle are two arrows and the word “lock”. One arrow points clockwise, the other is just an arrowhead opposite the nozzle and points away from it.
Ignore the clockwise arrow and “lock”
The arrow opposite the nozzle has to be aligned with the 1 on the collar. You may have to force the nozzle head around if it is worn. Just try the direction which is easier. The plastic stops inside that stop turning too far are very weak and probably broken any way, so once you’ve got it working just leave it in the unlocked position.
When the marks are exactly aligned you should be able to push the the nozzle head down and work the spray.
Only push down with one finger or thumb from the side where the arrowhead points, the opposite side where the nozzle is won’t go down, so the nozzle head goes down at a kind of angle.
The spray is inconsistent.
The other problem is that the nozzle may be encrusted and blocked. You can try holding the whole top assembly under a hot tap, maybe vinegar? soap? and poking inside the nozzle jet with a pin to unblock.
You may have to prise off the whole top to clean the assembly out properly. This is what I had to do to get it all to work.