If you have started the project build target on a lower API like 7 and want to change this to the API 15. Right-click the project and select “Properties”, then choice the “Android” from the menu. You can now select the target version and then select Apply or OK.
If this does not work try restarting Eclipse. Or if you’re running SVN, CVS, Team Foundation Server or some other version control make a check out of your project to make sure some of the files isn’t locked. For TFS right-click your project, select Team -> Check Out for Edit…
You can check out in the Team Synchonize Perspective as well.