how can I fix mybelling fridgefreezer typeHZS 3362 modelBK33 ART no 59544?

my belling fridgefreezer has a warm patch inside at the middle of the back pannel in the fridge compartment. The freezer part works fine but the fridge part is not cold enough. It is quite an old machine. Can you help?it is type HZS 3362, model BK 33 ,ARTno 595448,ser 8143004, 230V-50Hz 120w,Net volume 308,Refr r134A 0,167kg system compressor class1V, freezing capacity 5kg/24hr