Select all the songs you want to change using one of the following methods:
-Press Command-A to select all the songs in the library/playlist
-If the songs are all next to each other, click the first one, hold the shift key, and click the last one. All of the songs in between will be selected.
-Hold the Command (Apple) key and click each song individually that you want.
Then, press Command-i to Get Info. In the bottom right corner of the window that pops up, you can set the EQ for all those songs.