The parallels and meridians represent the coordinate system grid, called graticule. You can format the graticule using the Property Browser.
To change the Parallels or Meridians step:
Click the map item that you want to change. The selected map properties are listed in the Property Browser.
Expand the Parallels/Meridians property.
Select the Step property and set it to the desired value.
When you are done, press Enter.