Set-up a approbation workflow

Set-up a approbation workflow

Hi everyone, 

I can't find a way to enfore an approbation workflow, here is the use case:

--> If a sales assistant wants to change the account type from Active Customer to Old Client
--> this trigger a request for approval to the account owner
--> if the account owner approves, the change is made.

Thanks for your help, 

Tony