If there is a grub screw on the main body first release this or if there is a removable shroud pull this up then you should be able to pull off the swivel. Some cheaper taps just allows you to pull off the swivel. Once the swivel is off replace the "O" ring. Kits from B&Q. replace the ring and push back the swivel but add a bit of silicone grease or failing this washing up liquid.
BarryF
December 2009