How long has this been going on? There could be general Internet issues, or Mozilla may be having temporary server issues that could interfere with a download. Those should go away before long. If it's been a persistent problem...

Did you originally install this from an installer file downloaded directly from Thunderbird? Or from a "app store"? If the former, you can download the installer file for version 140.10.0esr from here:

https://www.thunderbird.net/en-US/thunderbird/all/

Choose the Extended Support Release channel and your operating system. Shut down Thunderbird, then install the new version right over the old.

Hello. The installer was done over a year ago from the Thunderbird website. All updates since then went flawlessly until just this last update which has been doing this for over a week now. Interestingly, my laptop updated to the current version without any problem. I'll try again and if worse comes to worse, I'll just download the new version installer and update it that way. Hopefully it won't do anything to the Profiles which contains all my contacts, emails, etc., etc.

Installing with the installer shouldn't affect your profiles, but if you want to be especially careful, you should make a backup copy of the Profiles folder before upgrading - it's trivial to restore them if necessary,

It's a good idea to back up your profiles regularly anyway, if your email is valuable and irreplaceable. I made myself a batch file that does it with a click.