You have to ask your employer to explain the discrepancy. On your tax return you have to enter the amount shown on your W-2, unless the employer gives you a corrected W-2.
Did you file your tax return before you got your W-2, so you filed with the wrong amount? If that's what you did, see VolvoGirl's reply below.
Did you already file using your paychecks? That's why we tell people to wait for the W2 and not file early. Your paystub won't match the W2. There are several items that affect the amounts. Most people that use the paystub have to send in an Amended return later.
Did you already go to the very end and hit the big orange Transmit Returns Now button?
You cannot change or add anything on the return that you just e-filed. It is too late, just like when you put an envelope in a US mailbox.
Now you have to wait until the IRS either rejects or accepts your return after Feb 12 when the IRS starts processing returns. If your return is Rejected you can go back into your account and fix it, then resubmit it.
But if the IRS Accepts your return you have to wait even longer for the IRS to fully process your return and you get the first Refund. You will still have to pay the tax due if you owe on it. THEN you can prepare an amended tax return.
Amend a 2020 Online return