How can I update SPF when a record already exists?

It is no longer required to include Postmark in your own custom SPF record.

Years ago when email standards were being formed, email providers used the SPF records of the From/Sender address’ domain to check for alignment. This meant that it was necessary for Postmark users to add a custom SPF record to their DNS in order to pass SPF alignment. This has since changed, and email providers no longer check the From field’s domain when evaluating SPF and determining the results. 

The Return-Path domain is now used by receiving email domains to check for SPF alignment. This means, with Return-Path verified, your emails sent through Postmark will always pass SPF by default, without any necessary action on your end, since the Return-Path of all emails sent through Postmark already includes our outbound sending IPs and SPF record. 

If you want to pass DMARC’s more strict SPF alignment requirements, you would need to set up a custom Return-Path so that the From address’ domain and the Return-Path’s domain match.