From IRS...
if you  received a stimulus check IN ERROR for a deceased relative or dependent child:

​Q41. What should I do to return an Economic Impact Payment (EIP)? (added May 6, 2020)
 
A41. You should return the payment as described below.
If the payment was a paper check:
  1. Write "Void" in the endorsement section on the back of the check.
  2. Mail the voided Treasury check immediately to the appropriate IRS location listed below.
  3. Don't staple, bend, or paper clip the check.
  4. Include a note stating the reason for returning the check. 
If the payment was a paper check and you have cashed it, or if the payment was a direct deposit:
  1. Submit a personal check, money order, etc., immediately to the appropriate IRS location listed below.
  2. Write on the check/money order made payable to “U.S. Treasury” and write 2020EIP, and the taxpayer identification number (social security number,  or individual taxpayer identification number) of the recipient of the check.
  3. Include a brief explanation of the reason for returning the EIP.

For your paper check, here are the IRS mailing addresses to use based on the state:
If you live in…
then mail to this address

District of Columbia, Idaho, Illinois, Pennsylvania, Rhode Island
IRS - Refund Inquiry Unit
2970 Market St
DP 3-L08-151
Philadelphia, PA 19104

Georgia, Iowa, Kansas, Kentucky, Virginia
Atlanta Refund Inquiry Unit
4800 Buford Hwy
Mail Stop 112
Chamblee, GA 30341

Florida, Louisiana, Mississippi, Oklahoma, Texas
Austin Refund Inquiry Unit
3651 S Interregional Hwy 35
Mail Stop 6542 
Austin, TX 78741

New York
Brookhaven Refund Inquiry Unit
5000 Corporate Ct.
Mail Stop 547
Holtsville, NY 11742

Alaska, Arizona, California, Colorado, Hawaii, Nevada, New Mexico, Oregon, Utah, Washington, Wisconsin, Wyoming
Fresno Refund Inquiry Unit
5045 E Butler Avenue
Mail Stop B2007
Fresno, CA 93888

Arkansas, Connecticut, Delaware, Indiana, Michigan, Minnesota, Missouri, Montana, Nebraska, New Jersey, Ohio, West Virginia
Kansas City Refund Inquiry Unit
333 W Pershing Rd
Mail Stop 6800, N-2
Kansas City, MO 64108

Alabama, North Carolina, North Dakota, South Carolina, South Dakota, Tennessee
Memphis Refund Inquiry Unit
5333 Getwell Rd Mail
Stop 8422  
Memphis, TN 38118

 
 

A foreign country, U.S. possession or territory*, or use an APO or FPO address, or file Form 2555 or 4563, or are a dual-status alien.
Austin Refund Inquiry Unit
3651 S Interregional Hwy 35
Mail Stop 6542 AUSC
Austin, TX 78741