Well, the very positive news is you're into the age of viability for your baby - if you had to deliver right now, your baby has a good chance of doing very well.
You can't move the placenta - as t_ says, it might move on its own.
The great thing is, your doctor is aware of this and if you start bleeding much more, your baby can be delivered and you can expect a good outcome.
I'm especially aware of placenta previa - my sister nearly died from blood loss, and she lost her baby daughter at 25 weeks. You are out of the clear for those outcomes - and can expect to have a healthy baby. A c-section at 37 weeks isn't really a huge concern - lots of women have c-sections (I had 3) and it's a blessing to have a healthy baby.
Best wishes.
I think it moves on its own. My mom had it and almost died with my brother because she had sex. She started to hemorrhage. Just take it easy, refrain from sex, do not stress out. Any sign of bleeding call your doctor. Everything will be fine:) my mom was even able to have abvaginal birth with my brother. He just came abliytle early but he did great and so did she.