drupal core needs regular updates - after backing up both drupal files and drupal database stages are as follows:
download latest version of drupal and copy the zip or tar.gz file to your server
unzip it - unzip drupal-8.7.7.zip was used to unzip the latest update
(or tar xzf drupal-8.7.7.tar.gz if you downloaded the tar file)
delete (or rename) the 'core' and 'vendor' folders
cut and paste the new core & vendor folders from the extracted zip archive
delete (or put in a new folder) all your top level files
cut & paste all the top level files from the extracted zip archive
run /update.php (for this site it would be https://boliston.co.uk/update.php