How can I clean a top plate of a steam press ?

If you have the brown paper bags you get from the grocery stores, lay it flat, an put table salt all over it. Then take the iron or press and rub it around on it. Off of course.:) Or just rub the salt on it. You should use only distilled water for the steam, if you have a steam dial. You can also use vinegar and wipe the plate part. That should remove hard water stains, calcium buildup and rust out of the steam holes. Hope that's what you were looking for.

just a thought I know it sounds silly but try a bit of tomato ketchup that gets things off saucepans try a little bit in a corner first my husband is good at things like that and will edit anything he comes up with when he comes in He suggests coco cola nearly as daft as mine but it might work

You could try bicarbonate of soda in warm water which should take of the brown bits.