From the object management settings for the object you want to set an override for, go to Buttons, Links, and Actions.
Click Edit next to the button or tab home page you want to override.
Pick the type of override you want associated with the action.
No Override (use default)—Use a custom override provided by an installed package. If there isn't one installed, the standard Salesforce behavior is used.
Standard Salesforce Page—This option is available only for subscribers who are overriding the actions on an installed custom object. If selected, the standard Salesforce behavior is used.
Custom S-Control—Use the behavior from an s-control.
Visualforce pages supersede s-controls. Organizations that haven’t previously used s-controls can’t create them. Existing s-controls are unaffected, and can still be edited.
Visualforce Page—Use the behavior from a Visualforce page.
Select the name of the s-control or Visualforce page you want to run when users click the button or tab.
When overriding buttons with a Visualforce page, you can select only Visualforce pages that use the standard controller for the object on which the button appears. For example, if you want to use a page to override the Edit button on accounts, the page markup must include the standardController="Account" attribute on the <apex:page> tag:
... page content here ...
When overriding tabs with a Visualforce page, you can select only Visualforce pages that use the standard list controller for that tab, pages with a custom controller, or pages with no controller.
When overriding lists with a Visualforce page, you can select only Visualforce pages that use a standard list controller.
When overriding the New button with a Visualforce page, you have the option to skip the record type selection page. If you choose this option, new records you create aren’t forwarded to the record type selection page. Salesforce assumes that your Visualforce page is already handling record types.
Optionally, enter any comments to note the reason for this change.
A standard button (New, Edit, View, Delete, and Clone) overridden with a Visualforce page doesn’t show up in Salesforce1 unless the Visualforce page is enabled for Salesforce mobile apps. Overriding standard list and tab controls isn’t supported in Salesforce1.