Hello again tam,
The form 521 itself it not was is used to increase your benefits. The form 521 is used to withdraw your current SS application, and then you can reapply for benefits which may now be higher since your age has increased. Here is an example of how this works.
Anyone who has applied for SS can also place a request to withdraw their application. If you applied five years ago and started receiving checks when you were 62, for example, you could file from SSA-521 to request a withdrawal of your application, and then after that’s processed you could apply for SSagain, this time at the much higher payments you would receive as the 67 year old that you now are.
The catch is that you have to repay any money you received. Which stands to reason, if you’re withdrawing your application five years after the fact you have to pay back those 5 years of SS checks. However, this can still be beneficial to you.
Essentially you have received an interest-free loan all these years. So now you repay those benefits and re-apply for the higher amount to which you are now entitled.
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Thank you tam and let me know if you have more questions.