Steps for Create a certificate for Android Market apk:
2.Now choose Android and then Export Android Application. In the next step confirm the project that you want to export.
3.Then click next and now you should be able to select create new keystore.
4.Now fill in the required fields and your should be able to sign your app.
5. Be sure to make a backup of the keystore file and remember your password. Losing this will make it impossible to update your application.
6.If you are using the terminal to create a keystore and you have Java SDK installed there should be a program called keytool in /usr/bin (on a unix based system).
7.On Windows the SDK should also come with the keytool but the install location may be different search for keytool.exe on your computer if keytool is not in your path already. With this tool you should be able to create a key in the following way:
8.Remember that once you lose your Certificate or it expires you will not be able to sign your application. Make sure that the expiration date is a long long time in the future.
keytool -genkey -v -keystore my-release-key.keystore -alias alias_name -keyalg RSA -validity 10000
Reference sites: Link1
Publishing Updates on Android Market :
2.When you publish an update to an application, users who have already installed the application may receive a notification that an update is available for the application. They can then choose to update the application to the latest version.
3.Before uploading the updated application, be sure that you have incremented the android:versionCode and android:versionName attributes in the element of the manifest file. Also, the package name must be the same as the existing version and the .apk file must be signed with the same private key.
Here we need to change:
Go to manifestfile and set the version code like this:
4.If the package name and signing certificate do not match those of the existing version, Market will consider it a new application, publish it as such, and will not offer it to existing users as an update.
Here , old versioncode: 1 and new version code:2
5.You have to have the same keystore file which you have used to upload the 1st version of application on android market. If you have lost this keystore file then you can't provide update to this application.
Note: Dont forgot to keep a backup of your keystore file.
Refered Links: Click Here