When I try to update my older version, I get a message saying there was a dpkg error. The log says

WARNING: The following packages cannot be authenticated!
advanced-clock-plugin
Authentication warning overridden.
(Reading database ... 37565 files and directories currently installed.)
Preparing to replace advanced-clock-plugin 0.7.1-0 (using .../advanced-clock-plugin_0.12.3_armel.deb) ...
Unpacking replacement advanced-clock-plugin ...
dpkg: error processing /var/cache/apt/archives/advanced-clock-plugin_0.12.3_armel.deb (--unpack):
trying to overwrite `/opt/maemo/usr/bin/advanced-clock-plugin-settings-ui', which is also in package advanced-clock-plugin-settings-ui
Errors were encountered while processing:
/var/cache/apt/archives/advanced-clock-plugin_0.12.3_armel.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

Suggestions? I tried deleting some files mentioned above, but I don't know how to do this properly.