You want to run the self-clean mode. Fill with water, then over a sink, with the iron facing down, press the self-clean button. Steam will eject, along with sediment and water deposits. You may have to repeat numerous times. The iron requires soft water, if you don't have soft water at the tap then buy bottled water at the grocery store to prevent this problem.
You can also buy a cleaning solution that you mix into the water reservoir, or you can make your own by filling the iron half-full of water then add white vinegar to the fill line, then perform the self-clean mode or steam mode over a sink.
Here is a Rowenta suggestions:
The iron is leaking.
Water: Use tap water in the iron. If tap water is hard or high in organic matter, we recommend using bottled spring water or mix tap water with distilled. Do NOT use 100% distilled water. Many softening systems may cause issues as well.
Auto-Off: If iron is equipped with auto off, you need to move iron or gently shake it an allow it to heat back up before beginning to iron.
Overuse of Burst of Steam: Allow more time between each burst of steam.
The Rowenta site does not list a manual for your older model, but here is their link: