The Bible says:
"Joseph took them both, Ephraim with his right hand toward Israel’s left, and Manasseh with his left hand toward Israel’s right, and brought them close to him. But Israel stretched out his right hand and laid it on the head of Ephraim, who was the younger, and his left hand on Manasseh’s head, crossing his hands, although Manasseh was the firstborn." (Genesis 48:13-14)
"When Joseph saw that his father laid his right hand on Ephraim’s head, it displeased him; and he grasped his father’s hand to remove it from Ephraim’s head to Manasseh’s head. Joseph said to his father, 'Not so, my father, for this one is the firstborn. Place your right hand on his head.' But his father refused and said, 'I know, my son, I know; he also will become a people and he also will be great. However, his younger brother shall be greater than he, and his descendants shall become a multitude of nations.'" (Genesis 48:17-19)
Can you see why Jacob did this?
The younger brother would be greater than the elder brother. So Jacob laid his right hand on Ephraim's head, who was younger. The right hand is usually the dominant hand and has more honor than the left hand. So Jacob laid his right hand on the younger son Ephraim, because God wanted to bless the younger son more.
Just as God blessed Jacob more than Esau (who was older), so God blessed Ephraim more than Manasseh.
Sometimes, God likes to turn things around and bless the younger person. God's blessing does not necessarily come to the one with the birthright. But God sees the hearts of all people and will choose a younger person if that person is faithful to God.
Similarly, God chose David, who was the youngest of all his brothers.
If your heart is faithful to God, God will choose you for good to do His will and to bless you no matter where you have been or what status you have.