By default, GIMP is free-to-use photo editing and creation software that is completely supported by donations. The software can do much more than pre-installed photo-editing tools, such as Microsoft Paint, but the developers do not include support to edit videos. A third-party add-on has been created that gives the software some video editing tools, but you'll have to edit one frame at a time.
Download and install Schumaml's video-editor third-party tool for GIMP.
Start GIMP from the desktop shortcut or from the Start menu.
Activate Schumaml's video editing add-in. You can find the tools for the add-in under the "Dockable Dialogs" window in the Windows menu.
Open the video you want to edit. As long as the add-in is installed, GIMP will allow you to open any .AVI file.
Edit any frames that need to be changed. Use the tools you activated to navigate frame-by-frame while making any appropriate changes.
Save the edited video.