how to download new theme, then upload new sidebar.php to blog

I’ve deleted a lot of code in my sidebar, so now the log-in for admin is gone. I set my blog up back in 2004, so it’s an old version. How do I grab a zip of the theme (or a different theme), download it to my hard drive, and upload the sidebar.php to my blog, so I can keep the main section of my blog where the posts and pages are located? If there’s a step by step ‘how to’ on wordpress, I’d be happy to take a look. As I’ve said, haven’t had to update or modify in four years.
Forgot wordpress version but it’s the wheatgrass theme.