Replacing

UnInstall the Add-in
Untick the add-in from the (Tools > Addins) dialog box
Close all the instances of Excel

Remove all Registry Entries

Open the Registry
The following registry entry contains all the add-ins that are currently loaded (when you have Excel open).

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Options 

In Excel 2016 and earlier, the following registry entry contains all the add-ins that are currently installed.

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Add-in Manager\ 

Replacing your Add-ins

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 Excel, but they do not have Excel open.
2) The add-in is available but is not installed within Excel (i.e. the check box in (Tools > Add-ins) is not ticked).