In Lyx, how to change "Part I" with some other text
LyX just provides an interface (or front end) to LaTeX. So, while you may see
Part I Some text
as your title inside the editor view, the LaTeX code associated with it
\part{Some text}
will output
Experiment I Some text
if you add
\renewcommand{\partname}{Experiment}
to your preamble (Document > Settings... > LaTeX Preamble). If you wish to change this in the editor as well (so the visual matches the PDF output), you need to edit the stdsections.inc
layout file; specifically the part style:
Style Part
Category Section
Margin Dynamic
LabelString "Part \thepart"
LabelType Counter
...
You would have to update LabelString
to read "Experiment \thepart"
instead.