From Test-Scratch-Wiki

Yes, you can! Simply click the blue "See inside" button on the project page, or open the project from My Stuff. After making edits and exiting the editor, the project will update automatically.

You can also upload an entirely separate project into a current one by accessing File > Upload from your computer in the editor and selecting and loading the desired project. Then, the project will keep its statistics and project id yet still change into an updated version. To get the file of your old project, go to your old project, and do the following: "File > Download to your computer.

Warning Warning: Your old project will be lost forever if you overwrite your project with a new one. To avoid overwriting your old project once you have edited it, use Save As a Copy before uploading the new project.

Another way is to move the scripts and costumes of another into your backpack, and drag them into your project.