After the download is complete, close all Thunderbird windows (Click Exit/Quit from the menu button on the right).
Delete the Thunderbird installation folder, which is located in one of these locations, by default:
C:\Program Files\Mozilla Thunderbird\
C:\Program Files (x86)\Mozilla Thunderbird\
Mac: Delete Thunderbird from the Applications folder.
Linux: If you installed Thunderbird with the distro-based package manager, you should use the same way to uninstall it - see Installing Thunderbird on Linux. If you downloaded and installed the binary package from the Thunderbird Download Page, simply remove the folder thunderbird in your home directory.
Now, go ahead and reinstall Thunderbird:
Double-click the downloaded installation file and go through the steps of the installation wizard.
Once the wizard is finished, choose to open Thunderbird after clicking the Finish button.
WARNING: Do not run Thunderbird's uninstaller or use a third party remover as part of this process, because that could permanently delete your Thunderbird data, including but not limited to, extensions, emails, personal settings and saved passwords. These cannot be recovered unless they have been backed up to an external device!