5 litres is not enough for a full ATF change that should include torque converter and cooling lines. But removing 5 litres of old and replacing it with new (including filter) is better than doing nothing.
Any decent gearbox specialist company or MB indi could do this for £200 or even less if you agree with them that you will buy the consumables. Depending on age and mileage I would recommend getting the (cheap) TCU plug changed at the same time. Loads on here and YouTube on this procedure. Insist they drain the torque converter and cooling lines, they will possibly charge more but it is a more complete job than just draining and removing the sump. Remember that its not just mechanical gubbins that run in the oil, the electrically operated solenoids and conductor plate are also swimming around in the same oil.