drupal core needs regular updates - after backing up both drupal files and drupal database stages are as follows:

1) put site into maintenance mode

2) download latest version of drupal and copy the zip file to your server

3) unzip it - unzip drupal-8.7.7.zip was used to unzip the latest update

4) delete (or rename) the 'core' and 'vendor' folders

5) cut and paste the new core & vendor folders from the extracted zip archive

6) delete (or put in a new folder) all your top level files

7) cut & paste all the top level files from the extracted zip archive

8) run /update.php (for this site it would be https://boliston.co.uk/update.php

9) take site out of maintenace mode