How do I fix this?

I have started this new project of mine (for mobile), and I have already added lots of stuff. The problem is, my project is a landscape, and now I found out that when opened on mobile, the project will appear with huge black bars below and above it instead of filling the entire screen, and I need to rotate the screen in order to accomplish this.

How do I make the project fill the entire screen without building the whole project tilted by 90 degrees?

This tutorial may help https://www.scirra.com/tutorials/73/sup ... reen-sizes

However, I would start setting "Fullscreen in browser " to "Sclae outer" (you can find this option in the Project Properties Bar) and adding to your background sprite the "anchor" behaviour and setting "Top edge" to "Window top" and "Bottom edge" to "Window bottom".