I'm considering upgrading to an Ultegra groupset, found a very good price online, but I do not know what bottom bracket is in my bike. I have a 2015 Giant Defy 2, aluminum frame, and to me, this looks like a press fit bottom bracket. However, the technical documentation that I got when I purchased the bike from a local bike shop says it has a Shimano BB4600, which is a threaded bottom bracket and looks nothing like what is on my bike so far as I can see. The only press-fit option with this groupset available is the BB71-41B. Can someone tell me if this will possibly work with my bike, or how I would go about determining compatibility? Additionally, since the crankset on the bike now is a Tiagra crankset, would it be possible to just swap it out with the ultegra crankset and leave the BB alone? Thanks.
Ultegra is still hollowtech II. It seems like the Defy 2 in the UK has a pressfit bottom bracket for hollowtech II cranks, while in the US it has an usual threaded hollowtech II bottom bracket. In any case, you need a hollowtech II compatible bottom bracket, which you already have (Tiagra is Hollowtech II only).
Just remove the current crankset and put in the ultegra crankset.
Aside: This guide from BikeRadar summarizes bottom bracket standards.