I sent my check into US Treasury with the wrong amount written in. The numeric amount was correct but the written amount was wrong. How do I rectify this?

Did you send too much or too little?

I wrote the numeric amount correctly, but put the wrong amount for less. I will need to send in more.

I mean the written amount was wrong

Has your check cleared?

The written amount was for less than the numeric was.

Right.  If it hasn't cleared then start with what Opus 17 said:  call your bank and see if they will honor the numerals you wrote.

If it has already been cashed for the lower amount then you can wait for a letter from the IRS requesting a payment for the difference plus an amount of interest they will specify (it won't be much) or you can go to the payments website Opus referenced and pay the difference.

If it hasn't cleared yet but the bank says they will go only with the lower amount, I wouldn't stop payment.  I would go to the payment website and make a payment for the difference.

I checked with the bank and they will only pay the written amount. Thanks for your replies and suggestions. I know I will have to do something, just not sure what or when.

Start by calling your bank and asking them how (or if) they will honor the check.  (I think my bank gives the numerals priority over the written amount.)

If it is not cashed yet, you could also stop payment to ensure that neither amount gets paid.  Then, go to the IRS payments web site and make a direct payment of the correct amount.  (The IRS may send you a bill for late payment interest but it won't be too much if you catch it soon.)  https://www.irs.gov/Payments