To straighten one's collar — idiomatically, to set oneself straight: to correct one's posture or attitude and adopt a more serious or disciplined demeanor.
面接の前に鏡で襟を正し、深呼吸してから部屋に入った。
Before the interview I straightened my collar in the mirror, took a deep breath, and then entered the room.
失敗した後、彼は襟を正して仕事に取り組み直した。
After failing, he pulled himself together and got back to work seriously.
先生は授業中に私語が多い生徒に向かって、襟を正すように言った。
The teacher told students who were talking too much in class to straighten up.