Are you talking vinyl, wood, hardiplank, etc?
If it is vinyl, then just snap the pieces out, cut lengths to match, and reinstall leaving the nails loose as you should see when you remove the panels.
To snap them out and back in, use a hooked tool (the right ones are pretty cheap at most Home improvement stores) under the bottom edge of a siding piece. You can find the bottom of a piece by checking at one of the vertical seams.
If it is vinyl, then just snap the pieces out, cut lengths to match, and reinstall leaving the nails loose as you should see when you remove the panels.
To snap them out and back in, use a hooked tool (the right ones are pretty cheap at most Home improvement stores) under the bottom edge of a siding piece. You can find the bottom of a piece by checking at one of the vertical seams.