If the back has small square or round 'cutouts' around the edge then you need a special jewelers spanner to remove it. Otherwise it may be a 'pop off' type which if you look closely around the edge will have a small gap which allows you to get the edge of a knife into it so you can prise the back off.
If your watch is a water resistant one then it may be wise to take it to a watch repairer as these can be difficult to get the back plate onto again without the correct tools.
If your watch is valuable then the cost of having a battery replaced by a professional is more than worth paying as you can easily damage the watch without the right tools.
Jon
December 2006