Originally posted 1 Jan 2014. Updated multiple times since then. Most recently updated with info for editing web.config on Windows Server. Our company website here at WordX.press (previously Fiddler.Online) had been a WordPress Multisite for some time. I decided it wasn’t really serving any good purpose to have it that way, partly because we’d simplified and only had 2 other sites running on the network. The other was a site we’d quit using and updating some time ago, but hadn’t yet removed. So I decided to roll it back to a single-site WordPress install. Here’s how it did it:
1. Backup Everything
I recommend a full cPanel backup to start with. Just go into your cPanel and click the “Backup Wizard” icon. Use the options for a full backup, then download the file when it’s completed. A separate backup of the database is also a good idea. From cPanel just go to phpMyAdmin, select the correct database, then the “Export” tab, and click the “Go” button. It’ll download the export.
NOTE: if you have or purchase BackupBuddy, you can actually use it to do all of this step, by backing up the entire Multisite in one neat
Source: https://managewp.org/articles/17375/how-to-revert-a-wordpress-multisite-to-a-single-site
Willie Chiu's Blog 