Deploy application with Express and Handlebars

Can someone help me?

I have an application in Express, Handlebars, and MySQL.

I want to deploy this application, but I can't. On some platforms, the application goes up, but the page gives a 404 error. This is because they are looking for a file called 'index.html' and in this case, the extension is Handlebars.

What should I do?

You really haven't given much info here but if index.handlebars works and index.html doesn't, then you have reconfigure apache or nginx or whatever you're using to serve index.handlebars first.