How can I deploy flexfields to a sandbox?
For value sets, key flexfields, and extensible flexfields, you must deploy changes directly to the mainline environment, not to a sandbox. But for descriptive flexfields, you must deploy your changes to a sandbox and test them before publishing your sandbox.
To deploy your descriptive flexfield to a sandbox:
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            Create or activate a sandbox, and select Flexfields from the list of tools. 
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            In the Setup and Maintenance work area, go to the Manage Descriptive Flexfields task 
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            Open the flexfield you want to edit, and make your changes. 
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            On the Actions menu, select Deploy Flexfield to Sandbox.