the rubber on my rear wiper blade has diminished in the middle, so the wiper doesnt actually wipe the rain. so, i need to change the rubber, is there any way i can do that rather than changing the whole arm? if yes, which type should i buy?

get any set from poundland then take blade out and cut to fit

Or Halfords sell wiper blade kits that you cut to length and fit to the existing arm. Bit pricier than Dave's suggestion though...