How do I make my seat stay upright on Peugeot 206 02 reg?

If arm release type it might need lubrication on the arm locking mechanism as it may not be locking in to upright position.
Remove the seat turn it upside down operate the arm to see what's happening. Lubricate moving parts. Put kitchen tissue down to catch lubricant drips so it doesn't stain fabric of seat.
Hope this helps

Bob UK
March 2017