How can I update the DKIM and SPF records to their default values?
You can use the WHM interface under DNS Functions »Enable DKIM/SPF Globally:
This can be also done by using the /usr/local/cpanel/scripts/enable_spf_dkim_globally script.
To execute the script, the -x flag has to be passed to the script; it also supports only running on a specific user:
/scripts/enable_spf_dkim_globally -x [--user=<user1>] [--user=<user2>] ... [--user=<userN>]