This security and maintenance release features 17 bug fixes in addition to 7 security fixes.
Because this is a security fix – please update your sites immediately.
You can also see a complete list of changes: https://core.trac.wordpress.org/query?milestone=5.4.1&group=component&col=id&col=summary&col=milestone&col=owner&col=type&col=status&col=priority&order=priority
If you have not done so already – download the update or simply use your existing installation of WordPress to update your site. Either way – this is an excellent time to make a backup copy of your site(s) before upgrading.