Batch edit one contact field (source) with same information
Personally I'd write a tiny php script but you could use the api explorer under civicrm/api: 


Chose entity contact and action get
Add your search criteria
Add option "limit" to something high enough (Defaults to 25) or combine with option "offset"
Hit "chain api call"
Chose entity contact and action create
Enter json like "{source: 'your source'}"
Run


You could test it with one contact by adding it's id in the get criteria.