Question

He said,"I must win the prize."​

Answer 1

★${\purple{\boxed{\huge{Question:-}}}}$

He said,“I must win the prize."

★${\purple{\boxed{\huge{Answer:-}}}}$

He said that he must(or had to) win the prize.

★${\red{\underline{\underline{\bold{Additional\: information:-}}}}}$

•How to change “must” for reported speech→

You don't have to change "must" for reported speech. If "must" means "have to" in the original, then you can change "must" to "had to" when reporting it.

${\green{\underline{\bold{Example:-}}}}$

•Direct speech:-

He said,“ I must work tomorrow.”

• Indirect speech:-

He said he had to [or must] work the next day.