I did not include the last pymt of the stimulus I need to change from 1200 to 1800 so how do I do that if I cant amend my taxes?

You cannot make changes to a return that is in pending or to a return that has just been accepted.  

We do not know if entering the wrong stimulus amount will delay your refund --- or for how long if it does.   The IRS has not offered any guidance yet on how they will handle returns that have incorrect stimulus amounts entered.  We all just have to wait and see.  You are among many people  who were confused and entered incorrect stimulus amounts, or who filed early and then received a check.   

The IRS should fix your return and add your stimulus so just wait.

The IRS says: If you make a mistake on the Line 30 amount, the IRS will calculate the correct amount of the Recovery Rebate Credit, make the correction to your tax return and continue processing your return. If a correction is needed, there may be a slight delay in processing your return and the IRS will send you a notice explaining any change made.

Related Resource:

• Errors: Will the IRS figure the credit or me on my 2020 tax return? What happens if I claim an incorrect amount?