it fails

You'd need to explain that in more detail.

Sorry but this is all I can tell you, starting Thunderbird a box opens telling me an update is available, I click on yes, and it goes into it's usual process, a box then opens informing me the update has failed and Thunderbird doesn't open. The only way I can open Thunderbird now is to click NO to the update.

Try to download the full installer from https://www.mozilla.org/en-US/thunderbird/ and run it.

You could try installing it afresh rather than going through the upgrade process.

https://www.mozilla.org/en-US/thunderbird/all/

Will installing afresh delete all my accounts and setting or will all these be preserved?

Everything will be preserved. Regardless of that, you should always have a recent backup of your Thunderbird profile folder. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring