First we check whether the panel is a standalone replacement or a sign the track, hinges, or springs are also compromised - on an older door, we're not going to hang a new panel on hardware that's about to fail next to it. Then we measure the section height and door width against what's currently installed, since older doors don't always match today's standard sizes exactly.
If a matching panel exists, we remove the fasteners and hinges holding the damaged section, lift it out without disturbing the neighboring panels, and set the replacement in, re-securing hinges and rollers before checking that the track alignment hasn't shifted.
One limitation worth saying plainly: we don't repaint or refinish panels to hide a color mismatch between old and new steel. If your door has faded over the decades, a brand-new panel next to older ones will show a visible line - we'll tell you that before you commit, not after.