If you get tax credits, you should have received a renewal pack in the post (if you haven’t you must let HMRC know right away).
The deadline for renewal is Tue 31 July, but don’t leave it till last minute. You may end up having a tougher time with long waits on the helpline. The easiest way to renew is online or through the HMRC app, which you can do until 11.59pm on 31 July. Or you can do it by phone until 10pm on 31 July.
https://www.gov.uk/renewing-your-tax-credits-claim
Also you must tell the tax office by 31 July 2018 if anything in your pack is incorrect. Your tax credits could stop or you could be fined if you do not.