How do I upgrade WordPress?

@kandeshopuk, you can’t as such undo like you might find in a word processing app for example. What you can do is restore your install if you’ve made a backup. Simply replace the existing files & database with the backed up version prior to the failed upgrade.

If you don’t have a suitable backup it could be worth while getting in touch with your website hosts, they should have a backups (how frequent these are is down to your hosts) they maybe a quick and easy answer to restoring your site to a suitable and stable version.