The Workflows tool allows you to set a contact property value for multiple contacts at once. Follow the steps in this article to set up a workflow for this purpose. Please note, the Workflows tool is only available to Professional and Enterprise customers.
It is also possible to change a contact property value for multiple contacts at once via an import. To do so, create a csv file with the email addresses of your contacts in one column and the new property values in the second column. See this article for more detailed import instructions. Importing email addresses that already exist in HubSpot will simply update the existing record with any new information.