The steps to upgrade a platinum card to a diamond card are as follows:

1. Log in to the official website of Guangxi Rural Credit Cooperative and enter the credit card center page.

2. Select the "Card Upgrade" option, fill out the upgrade application form, and submit the application.

3. After passing the review, bank staff will contact you and inform you of the specific information and procedures required for the upgrade.

4. Provide relevant information according to the instructions of the staff, sign the agreement, and complete the card upgrade application.

5. After completing the application, you will receive a new diamond card and can start enjoying the corresponding discounts and services.