how to update SSRS execution account automatically through power-shell?

Changing the passwords:

If you are using a Active Directory account you can use the ActiveDirectory PowerShell module.

Set-ADAccountPassword -Identity elisada -OldPassword (ConvertTo-SecureString -AsPlainText "p@ssw0rd" -Force) -NewPassword (ConvertTo-SecureString -AsPlainText "qwert@12345" -Force)

If you are using a Sql Server user than it would be using T-SQL

ALTER LOGIN UserName WITH PASSWORD = 'P@ssw0rd';  
    GO  

Updating the Service:

$service = gwmi win32_service -computer [computername] -filter "name='ReportServer'"
$service.change($null,$null,$null,$null,$null,$null,$null,"P@ssw0rd")