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As you probably know, Medicare has certain enrollment time periods, generally starting Jan. 1 and going through March 31 of each year. If you miss those then you have to wait for the next one to roll around. However, Medicare does have a special enrollment period if you are moving back into the country after living overseas and that time period lasts for two months after moving back. https://www.medicare.gov/sign-up-change-plans/when-can-i-join-a-health-or-drug-plan/special-circumstances/join-plan-special-circumstances.html#collapse-3195
Re-registering with Medicare is essentially the same as signing up for the first time. You can find instructions on that at this website: https://www.medicare.gov/people-like-me/new-to-medicare/getting-started-with-medicare.html
If you have additional questions when enrolling then on that same website, if you look on the menu bar to the right it will have a section where you can find someone in your state to talk to. For California it will take you to this link:
However, most of those links aren't much help unless it is a very specific issue.