Go to your local hardware store and buy a big oversize syringe and 6 ft of clear plastic tube the right size to fit neatly on the bleed nipple. Then start as suggested by Yorkshirekeith and suck the fluid out of the reservoir using the syringe until it's below the clutch master cylinder outlet but still above the outlet for the brakes. Then remove and replace the slave cylinder. There shouldn't be too much brake fluid mess because you drained the reservoir before you started but wash any away with water. Now stick the clear plastic tube onto the bleed nipple, put the end of the tube in some sort of container, and open the bleed nipple. Fill the reservoir to the top with new fluid and get a helper to push the clutch pedal right to the bottom and hold it there. Tighten the nipple and tell them to release the clutch pedal. Then loosen it and yell 'down', tighten it and yell 'up', and repeat for a fair old while until there are no more bubbles and nothing but clean new fluid is coming out the bleed nipple. Check the clutch is operating properly, clean up, and you're done. You may also want to do the brakes while you're at it, but be careful not to let any air into the brake circuit.