I'd be glad to help you update your client's address in QuickBooks Online. Also, I'd like to ask if you have receive any error message while on the process? This will help me narrow down the best resolution that fits your concern.
There are two ways to update the business address in QuickBooks Online. You can either change it in the Account and Settings menu, or customize your invoice' Content. This is how it looks like:
If you've encountered any error, I'd suggest using an incognito browser to check if this is a data-related issue. Then, if you can change it successfully, you can go back to the regular browser and clear the cache. This is done to clear the outdated data that is being accumulated. An alternative way is to use another supported browser.
Here are the shortcut keys to access a private browser:
Google Chrome: Ctrl + Shift + N
Microsoft Edge & Mozilla Firefox: Ctrl + Shift + P