You must renew your tax credits by 31 July 2016 if your renewal pack has a red line across the first page and it says 'reply now'. If you miss thedeadline your tax credits payments will stop. You'll be sent a statement and will have to pay back the tax credits you've been given since 6 April2016.
https://www.gov.uk/renewing-your-tax-credits-claim
https://www.gov.uk/renewing-your-tax-credits-claim