While in WHM navigate to SQL Services then phpMyAdmin. phpMyAdmin will probably open in a new window. Look at the top and towards the right for variables. Click on that then either scroll down to sql mode or use the filter. Hover over the row that says sql mode and an edit button should appear.
If an edit button does not appear you may not have permission to edit the value. If you have root access try logging in with that, if not you may have to submit a ticket to your Server Administrator.
This is one of the requirements of WHMCS.