You can change the headings on the marker information pop-up windows by editing the map.  Once you have selected a map to edit, click on the 'Markers' tab.   On this tab, there is a section labeled 'Marker Tab Options'.  This is where you can change both the content of the tabs a well as the headings.

In this case, just change the default values in the Tab 1 Header and Tab 2 header and the click 'Save' or 'Apply'.

NOTE:  You can change the content of the tabs from this tab as well.