Replacing

An add-in is no different to a normal file and therefore cannot be replaced while someone is using the add-in.
It is possible to replace the add-in if either of the following are true for All the users:
1) The add-in is installed within Word, but they do not have Word open.
2) The add-in is available but is not installed within Word (i.e. the check box in (Tools > Templates and Add-ins) is not ticked).