Trying to figure out where we can change the default owner of IATS-created account/individual and opportunity records

Hello, we are trying to deactivate the account of a user who has left our organization, and we are getting a "Error: This user is the default owner of records created by guest users and can’t be deactivated." message. Directions provided on the internet are fruitless because it leads us to site settings but we don't have any sites. I think it is referring to how this user is the default owner for NPSP/IATS records for donors and donations. But I cannot find where to change that owner so I can deactivate the account. Please help. Thank you! #NPSP #IATS #Record Owners