Hi @Rachel_Wu. This is actually something you will be able to do quite easily. If you pull the list of contacts that you have imported (Either via searching for them in a contact search, or navigating to Admin>DataCleanup> Look At Previous Imports) and choose all the contacts on the list, there will be a Mass Update Contacts feature under the Actions button.
Selecting this option will take you to a list of fields with checkboxes next to them. You will find Person Type under the ‘Standard Fields’ if you scroll down a little bit.
Select the checkbox next to Person Type, and choose the desired type, in the dropdown.
Then, you will scroll down to the bottom of the page and hit Process Action. This will set the Person Type to the selected list of contacts, in bulk.