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 at least 2-4 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 wrote: "I just got my receipt for a taxable donation I forgot about."
I assume you meant "tax-deductible donation" and not "taxable." Are you sure you can even claim it? You appear to have used Free Edition (is that right?), which does not have a Schedule A for itemized deductions on the Federal return. To claim a charitable donation on a Federal return requires Sched. A, which requires Deluxe or higher. AND one has to be able to itemize in the first place. Does the total of your itemizable deductions (including the charitable donation) exceed the standard deduction?
Or is this a state-specific charitable donation for some specific charity that is allowed on your state return? If so, we can tell you what you'll need to do, once you find out if the filed return was accepted or rejected.