Hi Andy,
That message usually means Microsoft AutoUpdate on a Mac needs a refresh, not that your Office apps installer is missing. Do a clean reinstall of Microsoft AutoUpdate, then run the updates again.
Quit all Office apps and Microsoft AutoUpdate.
Open Finder, then on the menu choose Go > Go to Folder. Type ~/Library/Preferences and press Return. Find com.microsoft.autoupdate2.plist and move it to Trash. Next choose Go > Go to Folder again, type ~/Library/Application Support/Microsoft and press Return. If you see a folder named MAU2.0, move it to Trash too. Restart your Mac.
After the restart open Word, on the menu choose Help > Check for Updates and let Microsoft AutoUpdate reinstall itself and then update Office.
If it still says that an upgrade is required, please paste the exact wording of the message and tell me your macOS version, for example Ventura 13.x or Sonoma 14.x. That will let me confirm whether the message is asking for a macOS upgrade rather than an Office update.