how to take the bottom plate off?

Without the specific model number of your Rowenta iron, it's not possible to give accurate advice. Generally, you'd be looking for a plastic cover on top of the handle that pops off to reveal screws underneath. You may have panel where the power cord enters the iron that would have to be removed to access further screws. You would then need to work from the top down and remove all the components to leave just the sole plate on it's own

Rupert
December 2019