Basically you just pop in the DVD and go ahead. As you have already installed 32Bit, I assume you know your way through the initial install procedure. Choose customise and then format the existing partition/hard disk.
But, before leaping in there, why don't you create another partition and put up a dual boot. You can then have the opportunity to examine both and decide which you prefer. The installation will automatically arrange the dual boot for you. Again, choose customised install and make sure you install to the empty partition.