I agree with Colum's advice.
Normally. when you delete a template from the RH explorer,
you'll be told that this will remove all references to the header
and footer in all topics. The template disappears from the list.
HOWEVER, the .htt file remains in the project.
If RH has properly kept track of things, you can re-import it
from the project folder.
If you try to re-import a template with the same name from
somewhere else, RH may object.
RH can get terribly confused if you add or delete files in
the Windows Explorer rather than within RH. More so when the
project is open, but even when RH is not running, you are taking
your chances when you meddle with Mother RH.
That's when you have to resort to Colum's suggestion to use
brute force on the apj file. Failing that, you may need to go into
the cpd file with MS Access and find a table to fix.
Harvey