Can you please provide more details? Screenshots may be useful as well. I don't know the steps you're taking, or even what you're trying to accomplish.
Libraries generally refer to what version of Android you're using--as opposed to adding a third-party JAR to provide specific functionality in your project.
If you are trying to change the Android build target (using 2.3, Gingerbread, instead of 2.2, Froyo, for example) then right-click the project in Project Explorer, click "Properties," then go to the "Android" section, select one of the checkboxes and click OK.
If you want to change a different library, follow the steps above, but instead of going to the "Android" section of the project properties window, go to "Java Build Path" and click "Add Library." If that isn't enough information for you, please state why you are trying to change the library, and what exact steps you are taking and what the result is.