How can I reinstall Microsoft office for Mac after I have changed the hard disk

If it is an older unsupported version of MS Office, then you cannot because Microsoft disabled the activation servers.

Otherwise you will need to reinstall a currently supported version of MS Office which may require paying for a new version or even a subscription. See these two Microsoft articles:

https://support.microsoft.com/en-us/office/upgrade-macos-to-continue-receiving-microsoft-365-and-office-for-mac-updates-16b8414f-08ec-4b24-8c91-10a918f649f8

https://support.microsoft.com/en-us/office/microsoft-office-and-the-mac-app-store-ad2293a8-f5aa-4652-ae3f-83dda906d748