Your employer should refund the amount paid. If not and you are not a US citizen or US resident alien then you will need to file either 843 or if eligible Form 8316.
IRS Form 843 - https://www.irs.gov/forms-pubs/about-form-843
IRS Form 8316 - https://www.irs.gov/pub/irs-pdf/f8316.pdf
Review this IRS website - https://www.irs.gov/individuals/international-taxpayers/social-security-tax-medicare-tax-and-self-em...
And this IRS website - https://www.irs.gov/individuals/international-taxpayers/aliens-employed-in-the-us-social-security-ta...
