If you are talking about when you start your browser to get on the web and it come up as your Yahoo page you need to go under the tools of your browser and reset it. For example if you are using Firefox, then you would go click on Tools (located at the top of the screen) and then click on options. Click on your general tab and then in the space that has Home Page beside of it, type in the url for Yahoo. That way when you open your browser it will have Yahoo as your home page.
If you want to get your old yahoo homepage back, you need to delete all the cookies, the cache, files and then restart the browser. However, if you are using IE you have to go internet options where you will delete the browsing history which will in turn erase, temporary internet files, history and the cookies.