Even though this is one of the simplest tips it is one of the hardest to find someone who has documented it.  So instead of just telling you I am going to take a quick sidebar first.  I wouldn’t want you to feel you got jipped by stopping here.

One thing that happened when I finished running the Mango update on my Samsung Focus was that the apps that I was testing forced me to uninstall them.  Now it turns out that this wasn’t that the SDK needed to be upgraded, but that the device had to be reregistered.  Fortunately Mango is backwards compatible as you would expect.

Now that we have that established if you have a project that you originally developed using the Windows Phone SDK 7.0 and want to upgrade it to SDK 7.1 simply go to the project properties and update the target OS version.

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It is just that simple.  Go ahead and deploy the changes and test them.  Then you can move onto creating proof of concepts with the latest Mango capabilities.