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Look on the backside of the wall area where the valve is locate and there should be a removeable panel that you access this from. You may have a drawer or other cupboard in there that you need to go through. Rare that they don't give you access. Some have a removeable piece on the front in the shower that holds the fixture. Those have caps over the screws that you remove and then take the screws out. Hope that proves helpful here. If you need more input from me please get back as all follow ups are included in our fee.
Hopefully, you got this resolved as I didn't hear back from you. You can always return to this posting again if you Bookmark it and we can do a follow up. All part of our service and please take time to enter a rating as that is the only way I get credit for my time spent helping you. Thanks! Be safe........