Unfortunately, no UI option exists within the Azure Portal to achieve this.  Updates will need to be made via an API call to your Azure environment.  Please see the linked Microsoft articles below, which covers what is needed, and how to perform the update.

 
To help you locate the required information needed, please follow the 'Find your SPN and tenant ID' steps below:

https://docs.microsoft.com/en-us/azure/cost-management-billing/manage/assign-roles-azure-service-principals#find-your-spn-and-tenant-id
 

Once you have located and taken note of the information above, please follow the instructions below to perform the permission update:

https://docs.microsoft.com/en-us/azure/cost-management-billing/manage/assign-roles-azure-service-principals#assign-enrollment-account-role-permission-to-the-spn
 
Please ensure that the update is attempted by a user who has an enrollment writer role.

The payload below can be used as a template for your convenience.

{
"properties": {
"principalId": "REPLACE WITH OBJECT ID",
"principalTenantId": "REPLACE WITH TENANT ID",
"roleDefinitionId": "/providers/Microsoft.Billing/billingAccounts/REPLACE WITH BILLING ACCOUNT NAME/billingRoleDefinitions/24f8edb6-1668-4659-b5e2-40bb5f3a7d7e"
}
} 

If assistance is needed with this process, please reach out to Support.