You cannot change or add anything on the return that you just e-filed, nor can you stop it. It is too late, just like when you put an envelope in a US mailbox on the corner.
If you left out a W-2, or a dependent, or a 1099 etc…DO NOT change your return while it is pending. The changes will go nowhere.
Now you have to wait until the IRS either rejects or accepts your return. If your return is rejected, you will be able to go into your account and make the necessary changes to your tax return and re-submit your return.
If the IRS accepts your return, however, then you have to wait longer until it has been fully processed and you have received your refund. THEN you can prepare an amended tax return and mail it in. You have to be able to work from that return exactly the way it was when it was e-filed originally. You will need to use a form called a 1040X. You cannot e-file an amended return. They have to be mailed, and it takes about 4 months for the IRS to process an amended return.
Meanwhile, DO NOT go in and start changing anything on your return in the system, or you will make a mess for yourself. Sit tight and wait until you see what the IRS does with the return you just e-filed
You can file an amended return after the IRS finishes processing the return you already submitted. It's okay to do this, I've actually done it twice. One time, I ended up owing more money, but I paid it.
Did you have to wait until you got your refund until you could do the amendments?
A filer SHOULD wait until he/she receives the Federal refund before amending a return, or he/she can end up with a big headache. The IRS could change something in the original return, the figures would then be different, and one could end up having to amend a second time (i.e., amend an amended return), which can be a nightmare to do in the software and could tie the amended return up for weeks.
Also follow the other recommendations that xmasbaby0 left in her answer above.