Step1: Goto "C:\Users\<user-name>\.android" (In fixed in Windows 7)
Step2: Delete "debug.keystore" file there or delete your debug certificate under
~/.android/debug.keystoreon Linux and Mac OS X.
Step3: The Eclipse plugin should then generate a new certificate when next time you try to build a debug package. You may need to clean and then build the project to generate the certificate.
The self-signed certificate used to sign your application in debug mode (the default on Eclipse/ADT and Ant builds) will have an expiration date of 365 days from its creation date.
Note: You cannot release your application to the public when signed with the debug certificate.