Custom Number Format for Chart

[deadrxn24]

Hello,

I needed a custom axis format for a chart I placed in Word. I right clicked on the axis then went to Format Axis > Number. I clicked on Custom and entered my format code. However, I accidentally added a wrong code.

My question is: how do I remove a format code from the type display window?
I've tried just highlighting the code and pressing backspace and delete, but neither attempt worked.

Thanks In Advance