According to the official Microsoft announcement, support for Office 2021 ended on October 13, 2026, and no further feature or security updates will be provided.

Please completely uninstall the original version and then install Office 2024. The steps are as follows:

1. Completely uninstall and remove the previously installed package: Uninstall Office for Mac | Microsoft Support

2. Reinstall Office 2024 and activate it using the activation tool.
   Download link for the installation package and activation tool: Office 2024 - SJTU Software Licensing Center