Changing cassettes?

Park Tool has an extensive library of instructions on bike maintenance online. 

Here is a link to the page on cassette removal.

http://www.parktool.com/repair/readhowto.asp?id=48

Read the section on Cassette Cog Lockring Removal and Installation.  You need a chain whip and a lockring tool to do the job.

You might want to get a book like Zinn and the Art of Road Bike Maintenance for general bike maintenance tips.

Ultegra cassettes with the same number of cogs are compatible with DuraAce. 

The choice of gearing is a function of your strength and the grade where you train and race.  The standard ranges for the rear cassette are typically, 12-25, 11-23, 12-27, etc.   The numbers are the number of teeth in the smallest and largest cog.  The 12-25 would be a common, middle of the road choice for a cassette.  The 11-23 is racy and is for flat terrain.  12-27 would be more for climbing.  Here is a link to the Shimano Product page for cassettes.

http://bike.shimano.com/catalog/cycle/products/component.jsp?PRODUCT%3C%3Eprd_id=845524441770452&FOLDER%3C%3Efolder_id=2534374302050611&ASSORTMENT%3C%3East_id=1408474395181674&bmUID=1178975714370

The front chain rings are typically 53 and 39.  With a 12-25 cassette, the largest gear ratio is the 53/12.  So for one turn of the crank, the rear wheel turns about 4.4 turns.  The lowest would be 39/25 which means the wheel turns about 1.5 turns per turn of the crank.  I can get really technical and mathematical at this point but probably you aren't interested in a physics lesson.  The best thing to do is to experiment a bit.  Get friendly with a bike mechanic or friend with some spare cassettes and try them out on a few of your standard rides.  The extremes of the gear rations are needed for going up and down hills.

TW 

TW