How can I update SPF when a record already exists?

  Sender Signatures, SPF and DKIM

Check to see if there are any other SPF records in your domain’s DNS. There can only be one SPF record per domain, so if there is an existing record just add to that record.

For example, if your domain already has the record:

v=spf1 a mx ~all

….then you would just add:

The final record would look like this:

v=spf1 a mx ~all