Rhe Cognito UI console does not allow you to change attribute values for individual users, which you may need for debugging. So you should do it using the amazon CLI utility.

Change standard picture attribute: 

aws cognito-idp admin-update-user-attributes --user-pool-id us-east-1_xxxxxxxx --username xxxxx --user-attributes Name=picture,Value=https://www.google.com.ua/images/branding/googlelogo/2x/googlelogo_color_272x92dp.png --region=us-east-1

Change custom param custom:first_name

aws cognito-idp admin-update-user-attributes --user-pool-id us-east-1_xxxxxxxx --username xxxxx  --user-attributes Name=custom:first_name,Value=Name --region=us-east-1
#amazon-cognito