You will see a notification on your phone when a new update is available. If you don't root you just have to wait and hope a new update is released for this phone.
I've read several places that HTC said 3.0 would only be put on newer, dual core phones (though only read that second hand in each case). However I used a variant of 3.0 in the form of the Virtuous Unity ROM and really liked it. Very nice looking.