Help, I crashed my site (Custom PHP)

Hi

I tried to enter some javascript in an echo tag in the custom php area to try and remove the error notifications on my site (I don’t want my client to have access to this).

The edit was set to be applied to the entire admin site… and it appears I made a mistake in my code. The admin panel crashed and I don’t have access to the plugin settings anymore in recovery mode. How do I change the php without reinstalling the plugin?