I appreciate you performing the steps to change a category name, @L1v1ngW0rd.

No matter what version of QuickBooks Online (QBO) you may have, the More drop-down button is always available. 

When QuickBooks has unexpected behavior, it is recommended to use a private browser. This is to verify if this is caused by the system or your browser. 

You can use the keyboard shortcuts below:

• Chrome: Ctrl+Shift+N
• Internet Explorer: Ctrl+Shift+P
• Firefox: Ctrl+Shift+
• Safari: Command+Shift+N

Once done, try to manage categories again. Check to see if it works. If they are, then your main browser may have too much historical data from previously visited websites which slows down internal processes and causes this kind of behavior.

To ensure this doesn't happen again, it's a good idea to clear the cache of the browser you're using. The following article contains the additional insight, as well as steps for the supported browsers: Delete or disable cache and temporary internet files in your web browser.

Let me know how it goes. I'll be looking forward to hearing back from you. Have a great day!