Removing a multi-site

I have decided to eliminate my multi-site and return to a regular WordPress site. I need to update WordPress. I updated a theme that is not compatible with current WP version and I don’t need the multi-sites any longer.

I have deleted all but one website from the account. I know just enough about web-building to be dangerous and this is a good example. I changed the code initially, but have no idea what I did. I am concerned about unintended consequences to my remaining site if I simply update WordPress, as normal.

Any suggestions? thanks