when you do your plot, instead of plotting all your y data against wavenumber, just plot against wavelength
plot(wavenumber,[y1,..,yn]) -> plot(wavelength,[y1,..,yn])
Or if you want to change the figure already created, you need to modify the Xdata property of all your plot with a command looking like
set(findall(gca,'Type','line'),'Xdata',wavelength)
where wavelength is the new vector you just calculated, which has the same size as wavenumber.