So if it was me, turn off the water
supply. Undo the 4 screws on the outside and whole unit should come out, might need to prise off the silcon sealant
. Then disconnect the hot and cold hoses and shower
hose. Use some cap blanks to seal them off, these are likely a metric thread. Take the unit apart, probably a grub screw that might need some PB blaster, inspect the housing for cracks. Take apart and replace seals/O ring. Reassemble. If the unit is stuffed you might only be able to replace with a US fitting and so need hose end adapters. Take unit to a hardware
store that sells both metric and SAE, Fawcetts Annapolis
may have something too, and find the bits/adapters that fit, takes the guesswork out.