i am not entirely sure, but have you tried cutex remover? another thing that might work, which is available at any chemist, is acetone, also used to remove artificial nails. otherwise, ask the person from whom you got or bought the henna.

just scrub it hard with soap and a facecloth. It wont come off completely but it will become very hard to notice. And be patient

Washing it a lot will help it to fade quicker, but too much scrubbing will just hurt your skin. Try washing with fresh lemon juice.

It will be gone in a week anyway.
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The outer layer of skin cells are stained. You can lighten it by exfoliating which is what the scrubbing is going to do but you can only do so much without irritation. You just have to wait it out for the rest to naturally fade off.

You can remove the outer dyed layer of skin by exfoliating or scrubbing. You can also help the dye fade a bit by using tooth whitening toothpaste on it, or by cleaning the bathroom and not wearing gloves the chemical cleaners will help the dye fade . No ammonia as that will make it darker.
REally , lemon juice helps the dye stay longer, so that isn’t going to help.
Soaking in a spa or hot tub or pool can also help.
Good luck