Welcome to the ODK forum, @Ami_ra! We're glad you're here. When you get a chance, please introduce yourself on this forum thread. I'd also encourage you to add a picture as your avatar because it helps build community!
At the top of the page, under the File menu, there should be an option to "Save Form to File..." which will export a file with .odkbuild
extension. Next time you can "Load Form from File..." to continue working in Build. However, I recommend you "Export to XLSForm" and then check out the documentation to author forms using that format. You can do everything inside of a spreadsheet, and there are more options available than in the Build web interface.