To give a group of invoices all the same description:

1. Create an invoice query for the invoices to be changed
2. In Administration > Globally change records > create a new Invoice change
3. Select the field for Description > Choose an operator of Add (and mark to overwrite existing)
Add the desired description
4. On the filters tab, change invoices from all to selected and select the saved query
4. Preview changes
5. Change now