there is an application(custom object).there are 2 record types for an application.I want to create a custom button on detail page on application which when clicked changes the record type of application.How to do this?

There are 3 objects- Account(std object),oppurtunity(std object),Application(custom object).Oppurtunity and application are linked via master detail relationship.I I have created 2 record types for account. I need to write a trigger on account so that when account record type is set to a specific value and client id(text field on account) is not null then status(picklist field on application )is automatically updated to a specific value.

There are 3 objects- Account(std object),oppurtunity(std object),Application(custom object).Oppurtunity and application are linked via master detail relationship.I I have created 2 record types for account. I need to write a trigger on account so that when account record type is set to a specific value and client id(text field on account) is not null then status(picklist field on application )is automatically updated to a specific value.