Hey @Mustakim_Billah welcome to the Community! Excited to hear you have been trying out the new app builder! 
On public link sharing: this is on our radar, but I don't have a timeline to share just yet. Darren shared more context on where things stand if you'd like to follow along, feel free to add a +1 on that thread as we're collecting requests to help prioritize the feature.
On converting back to classic: not right now. We only support the one-way path (classic to new app builder) since the goal is to move everyone toward the new builder over time, so there's no reverse converter today. For now, rebuilding in the classic builder is the only route if you need a public link in the meantime.
Let me know if that's clear, and happy to help with anything else. Thank you! 