Minecraft wiki entry for Creating_a_resource_pack, associated text: Open paint.net.
Do Ctrl+O and go to .minecraft/textures/block/ladder.png and double click.
Edit the Ladder however you want.
When you're done, press the X button in the top right of the window. It will open a dialog if you changed anything and ask if you want to save your work. Press Save then okay twice. You don't need to change anything in the menus.