I went and filed my taxes only to realize after they were sent that I reported my stimulus relief wrong. How do I fix it? Who do I contact?

I don't know if I was tired or what, but I accidentally put $600 in both spaces for received stimulus. I actually received $1200 the first time and $600 the second time checks were sent out. I've already filed and didn't realize my mistake until today. What can I do to fix this, and who do I contact?