Change Trust Policy in Serverless IAM Role (or any other params other than policy)

serverless-with-trust-policy-change.yml

## all other serverless.yml configuration
functions:
 # your functions
 
provider:
  name: aws
  # your provider config

resources:
  Resources:
    IamRoleLambdaExecution: # has to be this exact name, https://serverless.com/framework/docs/providers/aws/guide/resources/
      Type: AWS::IAM::Role
      Properties:
        AssumeRolePolicyDocument: # OVERWRITE assume role policy, rest is populated by serverless
          Version: "2012-10-17"
          Statement:
            - Effect: Allow
              Principal:
                Service:
                  - lambda.amazonaws.com
                AWS:
                  - arn:aws:iam::123456789:root
                  - arn:aws:iam::012345678:root
              Action: sts:AssumeRole