Choose a video editing tool: First, choose a video editing tool that suits you. There are some free tools to choose from, such as DaVinci Resolve, HitFilm Express and OpenShot. There are also some professional tools, such as Adobe Premiere Pro and Final Cut Pro.
Import Video: Open the video editing tool of your choice, and then import the video whose background you want to change. This can be done on the interface of the tool, usually using the "Import" or "Add Media" options.
Select a replacement background: In video editing tools, there is usually a way to select a background area that needs to be replaced. This may require the use of masks or selection tools to accurately select the background.
Import New Background: Then, import the image or video to be used as the new background. Please make sure that this new background matches the original video with the same size and resolution.
Replace background: Put the new background at the bottom of the original video. In video editing tools, there is usually a way to adjust the position and size of layers to ensure that the background matches the video content.
Adjust the transparency and blending mode: If necessary, you may need to adjust the transparency of the new background or use blending mode to better blend the background with the video content.
Export video: When you are satisfied with the video, you can export it as a new file. In the export options, select the desired video format and resolution.
Save: Finally, save your new video file.
Please note that changing the video background may take some time and skill, especially if you want to change the background to interact with the objects or characters in the original video in a complicated way. This may require more advanced video editing skills. If you are unfamiliar with video editing, you may need to spend some time studying and practicing, or consider seeking professional video editing help.