STEP 1 : Login to cPanel ( Guide here )

STEP 2 : Navigate to Domains -> Zone Editor

STEP 3 : Filter by TXT Records

STEP 4 : Add Or Edit the DKIM / SPF / TXT Record

See if there is any existing SPF/DKIM/TXT Records. You will notice it by the v=spf1 ( SPF Record ), or v=DKIM ( DKIM Record )

Example of SPF Record :

Example of DKIM Record :

If it exists, click on EDIT, make the edit to the record and click Save Record after editing the record. Edit the name and Value according to the custom SPF/DKIM Record you want.

Example of SPF Record Edit :

Example of DKIM Record Edit :

If the SPF/DKIM Record does not exist, you can create a new one, simply by clicking + Add Record dropdown menu and selecting Add "TXT" Record

Example below :

Fill up the valid zone name and text value according to your custom DKIM/SPF/TXT Record and click Add Record once you are done.

Please allow up to 24 hours for the DNS to propogate. Please contact support if you have further questions, either by live chat or ticket support