Opening to piano up is not risky...but do not touch or remove the action
That's the sternest I can be...
Now to open the piano (I'm assuming that you want to open the bottom panel...
Reach under the keys in the middle and feel for a metal spring clip at the top of the bottom panel. Push that up, and the panel should be able to move towards you and then you can either pull it straight back, or out the side (if there are legs in the way) The only place you need top be carefyul here, is if the panel needs to go to the side, otherwise you may scratch the legs.
Your pedals should now be in plain sight. find out why they are stuck. follow each mechanism all the way intill they dissapear to find out what the blackeage is. If you can, try to fix it.
To put the panel back on, make sure to line up the little pegs in the bottom of the panel, and then just clip it back on!
Good luck
Topic: Ahhhhh Rental Pianos