You can copy your site to a new URL and use this new site for trying things out. Theoretically you should be able to copy this site back to the original site, but this sometimes doesn't work right so I recommend copying the changes and pasting them back.
Unfortunately when some people try to permanently delete the site to free the URL, they get a mysterious "Server error" and thus it is impossible to move "mysitetemp" to "mysite" because the URL is not available.
This is a known issue. There is no known workaround apart from waiting 30 days after which the site will be permanently deleted. See:
If this happens you you should be able to recover the site. If you access the URL of the site while signed in with the same account that you used when you deleted it you will see:
You will have buttons to recover the site or cancel.