I think you will find uninstalling either of the version windows tell you are installed will simply uninstall Thunderbird. The windows application manager is notorious for getting what is installed wrong.

So why can you not install the update?

Windows control panel, installed programs, indicates there are two versions of Thunderbird installed. When Thunderbird tried to update, I received a warning from Thunderbird that the two versions existed and I verify that to be true.

Can I safely uninstall the older version without losing all my emails and data?

Just uninstall one and see if Thunderbird work after I suppose. the setting and data are not involved in installing and uninstalling so I am somewhat blasé about doing it.