the 42 tooth ring is a front chainring. Your bike probably is coming with a 39 tooth inner ring (or possibly a 42 already) Going to a bigger inner front chainring will raise, not lower your gear ratio. If you want lower gearing, first check and see if you've got a 39 on the front. If not, you might want to buy a 39 inner ring. If you want lower than 39x25, you can get a 12-27 ultegra cassette that will give you a slightly lower gear. I don't think a standard Ultegra derailleur will shift bigger than a 27 rear cog.