First, you'll need a family tree software program into which you can import/export data. You can get a plain vanilla program called PAF at the LDS site here...http://www.familysearch.org/eng/default.asp?page=home/welcom...
Once you've installed your program...
1. Download your member tree as a gedcom file
2. Import that file into your program
3. Within the program, delete all entries that you don't want in your new member tree.
4. Save and upload the now-trimmed family tree gedcom file
to ACOM as a new member tree
That's one way of doing it.
Another way (perhaps easier) is to download your current tree then upload that same file again giving it a new name. From within the tree, you can then delete unwanted entries.
Whatever you decide, delete unwanted lineages carefully or you'll probably wind up with unconnected remnants that can be a pain to ferret out for deletion.