You can’t directly edit 32-bit color icons but you can convert them manually to 24-bit :
Right click > Add new icon
Open your Icon1.ico file.
Right click > New Image Type or press Ins to open the New Icon Image Type dialog.
Select the format you want, say 96x96, 24 bit or add any custom size and color depth.
Then copy/paste from your 32-bit icon file and save.
Et voilà ! You can now edit your 24-bit color icon.