The only way I have tried is to work on the new site on a different server, or it can even be done on a subdirectory of your live site, so you can just leave the live site as is. Once done, just migrate over the site to the live server/directory and that’s it.
I think there are plugins that offer this kind of functionality as well, being able to work on a new theme while the live site uses another, but I’m not really sure.
This isn’t really a BB specific question, so I’ll just leave this out for the community. Feel free to ask the same question over to our BB FB Group and/or Slack Channel. We’ve a lot of active users there so you’re most likely to receive more recommendations there, if ever, than here.
Can you, instead, send us an email with the issue at hand via our helpdesk, hello [at] wpbeaverbuilder [dot] com, and we’ll take it from there?