How can I change the plan to 5GB/$30 per mo

Hi there:

I will be in US 1 week later, and I just heard from others about 5GB/$30 plan. Now I am using $50/mo unlimited plan and I want to change to that plan. Unfortunately I can not find the way to change the plan on T-mo's website. Can you tell me the steps to do that. Anyway, I do not mind change my phone number if necessary.

Best

Ji