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.   

You may not need to amend your return.  You will need to wait for you return to be processed. 

The IRS has not published any guidance on how they will handle a misstatement of stimulus amounts on the 2020 tax returns.  They may catch the error and notify you of the correction. 

Due to the timing of the stimulus payments and the tax filing season, this experience is very common.  

Here is a link to monitor for updates on the Recovery Rebate Credit.

[edited 2/22/2021 8:33am EST]