I suggest you wait for the automatic upgrade. Doing so provides the best possibility for success.

Take a backup of your Thunderbird profile folder just in case, then make sure to read the v78 release notes here https://www.thunderbird.net/en-US/thunderbird/78.2.1/releasenotes/ and the blog announcement here https://blog.thunderbird.net/2020/07/whats-new-in-thunderbird-78/ and make sure you understand the advisories issued there, especially regarding add-ons. When you're ready, download it from thunderbird.net. Your emails and contacts should show up just fine. Make a habit of regularly backing up your profile folder, because not even Mozilla can guarantee you that anything won't go wrong for any reason at any given time. Your data is your responsibility.