File permissions in WordPress

I have completed the migration of an existing web-site to a new host on a VPS server that is self managed.

In the site health check comes up with a couple of errors:
1) The?wp-content/upgrade-temp-backup?directory exists but is not writable. This directory is used to improve the stability of plugin and theme updates. Please make sure the server has write permissions to this directory

2)

Some files are not writable by WordPress:

• xmlrpc.php
• wp-blog-header.php
• readme.html
• wp-signup.php
• index.php
• wp-cron.php
• wp-config-sample.php
• wp-login.php
• wp-settings.php
• license.txt
• wp-mail.php
• wp-links-opml.php
• wp-load.php
• wp-includes/class-wp-styles.php
• wp-includes/class-wp-user-query.php
• wp-includes/l10n.php
• wp-includes/date.php
• wp-includes/php-compat/readonly.php
• wp-includes/class-wp-oembed.php
• wp-includes/images/w-logo-blue-white-bg.png

Normally fixing these issue is a matter of changing rights and ownership from the command line, which is what I have attempted using:
find /home/admin/web/my_web_site_folder/ -type d -exec chmod 755 {} \;
find /home/admin/web/my_web_site_folder/ -type f -exec chmod 644 {} \;

This has not fixed the problems. In addition, there are other problems which are probably symptomatic of the same issue. It is not possible to upload images to the site via Workpress (although it’s no problem with FTP) and plugins will not update, failing with an error “Unable to create folder xxxx”.

I am also unable to update to the latest version of WordPress, which also fails with an error that it is not possible to create the folder.

Please help.

The page I need help with: [log in to see the link]