The settings grid is displayed in a categorized grid.
To view the setting in a category you can click the + to expand and the - to collapse.
A setting is enabled if it's enabled check box is checked, you change a value by editing it's value directly in the grid.
After you have made changes the save to server icon becomes enabled,
and after you have saved, the reload button become enabled.
The green + and red - will expand and collapse the whole grid.
See also:
Server Configuration (postgresql.conf)