Upgrade Sony Ericsson XPERIA Mini ST15i from 2.3 to 4.0 topic

Howzit going guys.Im in the process of UPDATING my old Sony Xperia mini X10 (served me well for 5 years) to a newer Sony Ericsson XPERIA Mini ST15i

My question is, Do I just plug it in to my Sony companion and it will upgrade by itself or do I have to do some rooting and stuff?

According to Sony Ericsson XPERIA Mini ST15i specs it is running on 2.3(Gingerbread) right now but upgradeable to 4.0 ( I beleive Ice cream sandwich)

My main purpose is to maybe speed it up a bit plus theres so many apps that requires Android 3.0 and above so just staying with 2.3 my apps are limited

Any help is greatly appreciated

Thanks folks :D