How do i straighten a penis which has a curvature that was forced gradually in childhood?

You can't!: Your penis is either curved naturally or due to trauma or by a process called peyronie's disease. If the curvature is significant, cauising difficulties, painful or if there are other issues, see either your pcp or a urologist. If significant, surgery may be necessary. I am not aware of anything you can do to straighten the curvature.

Consult urologist: An erect penis should be straight, and if it is not, there may be congenital scar tissue externally that is holding the penis curved, called chordee. Peyronies disease is caused by scar tissue internally that does not allow the penis to straighten. Trauma to the penis can also cause internal injury. A urologist can assess and recommend medication or possible surgery.