I would like to change the font-size of the as-menu, as of now it is 18px, how can do that?

Where can I find this below code (inspect element showed me the below code for as-menu) and edit the font-size? Please let me know

#as-menu ul.as-menu > li > a, #as-menu ul.as-menu > li > span {
font-size: 18px;
font-family: Arial, Helvetica, sans-serif;
color: #;