I have a live WordPress site with a coming soon page template currently active. How can I develop the website on a sandbox and then upload all changes to the live site? The sandbox has to be on the same live server, not through local development.
I have the wp-admin access but I don't have cpanel access. The server is through shared hosting.
You can't in any meaningful way put up a "sandbox" given those conditions. You have no way to isolate the code, and you need to isolate the code somehow. As I wrote in a comment, you are putting probably buggy, probably unstable, and potentially insecure code on a production server. You need to isolate-- "sandbox"-- the code where it can't effect anything outside of itself and that means a subdomain, a seperated domain, an independent database, and probably other precautions as well. If you are asking "how" you don't have the knowledge to do this safely.
Pay for another hosting package or develop locally.
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