An adult passport renewal is what you need - from the IPS site:
Young people and adult passports
Application for a young person over the age of 16
When a child reaches 16 they can apply for a first adult passport if either of the following is true:
•they never had a child passport
•they were included on a parent's passport rather than having a child passport of their own
If they have a child passport they can continue to use it until it runs out. The next passport they apply for should be an adult passport renewal.