Repairing bath tap/ shower mixer

[Rollin]

The bath taps in our bathroom have an outlet so that water can be diverted to a shower head. To divert the water, you push down on a ring that goes around the shower outlet. Unfortunately, this has stopped working properly and now when pushed, water is directed half to the shower and half to the taps. I don't want to replace the whole thing as the bathroom will be renewed early next year.
How would I go about repairing it?