If you're logged in to Speedify with an account (as opposed to just using the free, not logged in version), you can access information about your Account by clicking on your email address from main screen. Once you've done so you'll see a few options available to you:
Manage Subscription: This option is only presented to users who purchased from the Speedify web site. By clicking this option you can choose to edit your subscription or cancel your subscription. Users who purchased from either the App Store or the Play Store, can change their settings from those stores directly.
Change Password: You can change your Speedify password here.
Back on the Main Screen you should see the following:
Switch Accounts: Clicking this option will log you out of Speedify, so that you can switch accounts.
Upgrade: Clicking this option will offer you some purchasing options for Speedify plans.