It should just be "sudo apt-get install lxde" from what I remember.
Then logout, and you should see a dropdown menu with gdm3 or something similar, just select lxde there and log back in.
Make sure you install grafs kernel if you haven't already, there are a few compiled ones in a thread here already with instructions on how to install it, it will be a pretty good speed boost.
STOP! Before you ask questions... READ the first post! RTFFP = Read The F*cking First Post
Yes, A 3.55 "jb" (cfw) is available, no >3.55 "jb" or cfw yet.
Yes, A 3.55 downgrade is available. But not to lower than stock firmware! (What it came with)
No >3.55 downgrade is out there yet.