The indirect form of the sentence "He said to me, "You had better take an umbrella, it is raining heavily." is-

He advised me to take an umbrella as it was raining heavily.

He told me better for taking an umbrella as it was raining heavily.

He advised me to take an umbrella and it was raining heavily.

As it is raining heavily, he told me to take an umbrella.

As it was raining heavily, he advised me for taking an umbrella.