HP-01... how to open it?

[Gianmaria (Italy)]

I bought a nice HP-01 gold version, it's a nice exemplar complete with stylus pen, presentation case and Owner's Manual. The watch is clean without major scratches but the keys are a little dirt, so numbers and symbols pictured inside aren't really bright and clear.

I am wondering if someone know the clue to disassemble it to have access to the keys and to clean them and may be to repaint the white symbols.

Thanks!