You will be prompted to sign a transaction in Metamask. This transaction will give the new RNDR contract permission to move your tokens.
Do NOT refresh the page after this transaction. Step 2 will become available after the Metamask transaction is processed (please note times will vary).

You will be prompted to sign a transaction in Metamask. This transaction instructs the new RNDR contract to perform the migration, setting your balance on the old contract to 0 and your balance on the new contract equal to your original balance.
This button will become enabled once your transaction from Step 1 is processed.