If you have already set an email, you can only change your email through rec.net.
See this guide for how to change your email through rec.net.
Changing your email address through rec.net
1. Visit http://rec.net and log in using your username/email and password. Once you have logged in, click your profile on the top right to visit your profile. 
2. Click the Settings wheel below the account name. 
3. Visit the Email tab in the settings page and input your email and press "Update Email". You will receive a confirmation inbox in your inbox, make sure to accept it.
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