To change the fork oil on a Derbi Senda 50, the first step is to remove the fork from the handlebar stem. You can do this by lifting the bike onto a central stand to facilitate access to the bottom of the fork. Once the bike is lifted, remove the fixing screws that hold the fork to the handlebar stem using the appropriate tool. Make sure to keep the bike stable while removing the screws. Once the fork has been removed from the handlebar stem, you can proceed to drain the old oil. Find the drain plug at the bottom end of the fork and unscrew it to allow the oil to drain completely. Make sure to place a collection container under the fork to collect the used oil. Once the oil is completely drained, screw the drain plug back in. Next, fill the fork with new oil up to the level recommended by the manufacturer. Make sure to use the specified oil in the owner's manual of your bike. After filling the fork with new oil, reattach the fork to the handlebar stem using the previously removed fixing screws. Make sure to tighten the screws well to avoid oil leaks. Finally, make sure the bike is stable on the central stand before lowering it. Once this process is completed, you will have successfully changed the fork oil on your Derbi Senda 50.