In the Quantum Composer, there is always a Switch to Qasm Editor button:

Click the button and you will get a warning that working in the editor could affect your quantum score, click OK:

Then you are brought into the OpenQASM editor:

The editor helpfully has a Quantum Composer view of the circuit on the right, but you are no longer able to drag and drop gates. Instead, you can edit the OpenQASM directly, and see the modifications in the Quantum Composer, or import OpenQASM from an external file on your computer. Once you are happy with your circuit, the view gives you the opportunity to download the OpenQASM via the Download button as well.

Let's edit our circuit to change it from representing 10,000 to 11,000 by adding in an X gate onto the qubit labeled q[1]. We do this by adding x q[1]; to the code in between the register declarations and the measurements. As soon as this line is added, the user interface updates to display the modified quantum score...