I have a 9month old cat who I trained myself and she was very good but recently she has started not to use her litter tray and go in two usual spots. I have another cat around 3yrs old and have two separate litter trays how can I train her back?

The first thing to determine is if it is a physical problem.
Is she urinating outside the box?
If so, it is possible that she could have a urinary tract infection.
I would take her in to see the vet to check out possible physical reasons and go from there.

As mentioned above, always always ALWAYS rule out medical problems first. I would book an appointment with your vet to rule out an infection.

If that is ruled out, you need to do a bit of trial and error. Did you change the litter? How often do you change it?
Technically there should be 1 box for each cat plus one, so two cats should have three boxes. If that is not viable ensure the boxes are being cleaned regularly (I clean mine twice a day for two cats).

Get back to us on this. I went through extensive issues with inappropriate urination!