The Update software will be available depending on the specific model and the carrier (operator). To view the current availability upgrade kit you can visit http://talk.sonymobile.com.
You need a PC or Mac to update your Xperia. Follow the instruction in http://www.sonymobile.com/update. You may be prompted to download a PC Companion software or a Mac Bridge. Upgrade procedure for Mac users is a bit more complicated but you wouldn't missed the upgrade right?
Other models like the Xperia arc, Xperia PLAY, Xperia neo, Xperia mini, Xperia mini pro, Xperia pro, Xperia active dan Sony Ericsson Live with Walkman will get their own scoop respectively from the end of May or early June.
Some models have slight software differences inside. It depends on the product, the country and the operator who provides it. Sony uses the identification number called the Sales Item (SI). SI is an 8 digit number in the format of XXXX-XXXX. You can see the SI code under the battery after you open the back cover. You can see if an update is available to match this with SI code table on this page http://talk.sonymobile.com.
See the picture below.
Source: sonymobile's blog, phonearena