I have a family of 4 and we file jointly. The first stimulus was $2,900 instead of $3,400. The second stimulus was $1,800 instead of $2,400. How do I get the difference?

When starting my 2020 tax's I found the second that asked about how much money we received for stimulus.  I assumed it only meant for the first check and put $2,900 in for the amount and it said that the amount looked right.  My family does not make for the 75,00 a year how can that be right?  I'm trying to find out how I get the additional money from both stimulus checks.