Global editing of CiviCRM fields
This is possible using Update contact using profile. Below are the step to achieve this


Create a Profile with Contact Sub-type field.



Do Contact Search and select contact(s) you want to update and select 'Update multiple contacts' from Actions dropdown.





You will be redirected to Profile selection page. Select the profile you had created at #1 from 'select profile' dropdown and click on continue.





You will be redirected to a page where you wii see fields against each contact. Select the contact subtype for each contact and then click on Update Contacts. You can also click on Copy image (besides column title) to copy the content of 1st field into rest of fields beneath it.




HTH

Pradeep