When uninstalling Thunderbird (or Firefox), all so-called "profile data" should be retained. That means you can safely uninstall and reinstall Thunderbird without user data, including mail. Technically you should both be able to install 45 on top of 38 or uninstall 38 first and install 45 without any issues. However, I would prefer to uninstall 38 first and then install 45. When uninstalling, you should not be prompted to remove any profile data - this only happens when starting the Profile Manager, if you know (or found) how to do so. Nevertheless, it is always good to [[Profiles - Where Thunderbird stores your messages and other user data|create a backup of your profile data]] in case anything goes wrong, even occasionally/periodically. Please do not use MozBackup for this, since it is no longer supported and people have reported issues doing so. Does this help?