If you're on 1.4, you probably haven't seen our new install script yet? It makes things a lot easier - no more editing of files.
Read the instructions
, including those that come up when you call up the install script. Of course, it never hurts to have backups of everything.
Your theme will have to be updated. See the list of changes
. Depending on how customized your is, it may be easier to start over by customizing the Professional theme that comes with 1.6.1.