You do not have to do anything. The newly issued Medicare cards no longer require a signature. The new cards are part of the 2019 reissue and have a number of new features to make the process of using them a lot easier. One feature is that you no longer need to add you signature to the card.
The reason for the lack of signature is an attempt to make the new cards more convenient. If you notice the new card is smaller than its older counterpart so it can fit easier in your wallet. The smaller design also makes the card easier for your medical practitioners to read. The new cards are also bilingual to avoid regional drawbacks.
The other feature of the new Medicare card is that your social security number has been replaced with a Medicare number. The new number is designed to prevent identity theft and allows for more secure usage of your policy. Everything else of the card remains basically the same. The make-up of the new card does not affect policy at all, however, so expect to appreciate the same coverage as always.