Now when he was in affliction, he implored the Lord his God, and humbled himself greatly before the God of his fathers, and prayed to Him; and He received his entreaty, heard his supplication, and brought him back to Jerusalem into his kingdom. Then Manasseh knew that the Lord was God. 2 Chronicles 33:12-13
Manasseh later removed all the idols and stopped many of the sinful practices that the Israelites had been doing. He humbled himself. In this case it means he recognized his sin, recognized the fact that God is God and he hadn’t been listening to what God was telling him. Today this means reading His Word, listening to teachings, and applying it to our lives.