Several answers from Joel suggest that it is possible to 'republish' a tree. How is this done? Some people seem to suggest deleting the existing tree and then re-importing a new Gedcom - but they also suggest that the re-import does not work so I am reluctant to try this.
The simplest way to republish your tree is simply by publishing it again from the FTB, under the same name, to the same site, while logged in with the same member that you used when you initially published it.
It is also possible to delete the tree from the site (in family tree --> Summary page) and then publish it as a new tree, however this is not necessary.
In both cases, your data should not be harmed.
If you do encounter any sort of problems, please contact us at firstname.lastname@example.org