There are 5 or 6 adjusting bolts across the back of the head that control the sickle angle. You loosen the nut that sandwiches the flex head parallel arms. Then turn the bolt out to decrease the sickle angle.
In the picture of the frame they are" adjusting bolt #12 and bar/nut that it goes through #10. They are on the back bottom of the header.
After you have adjusted these to where you want them you then have to adjust the sickle gear case stabilizer spring.( you do this with the header on the ground.) In the picture you want the spring # 8 to be 2 1/2 inches long with the header on the ground. Then there should be 1/4 inch cap between bolt head #13 and pivot # 12.