If your rest message contains multiple records then you will need to iterate through each record,
You can do this utilizing a scripted rest api (and import sets)
Tony Vecchgio has provided a very helpful guide
https://www.youtube.com/watch?v=DyIuma1cZrg
https://community.servicenow.com/community?id=community_article&sys_id=d5c94462dbf557002328f3231f961...
You need a unique identifier for the record, and for tasks this could be the task number or sys_id,
for other record types this may be a single field match or a combination of fields, as long as the result is always 1 to 1 match.
SOAP would also allow multiple record updates, but I believe that scripted REST is the recommended solution.
