For standard objects, from Setup, click Customize, select the appropriate object or tab link, then click Buttons, Links, and Actions.
For custom objects, from Setup, click Create | Objects, and select one of the custom objects in the list.
Click Edit next to the button or tab home page you want to override.
Since events and tasks don't have their own tabs, you can only override their standard buttons and links.
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.
S-controls have been superseded by Visualforce pages. 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, only Visualforce pages that use the standard controller for the object on which the button appears can be selected. 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, only Visualforce pages that use the standard list controller for that tab, pages with a custom controller, or pages with no controller can be selected.
When overriding lists with a Visualforce page, only Visualforce pages that use a standard list controller can be selected.
When overriding the New button with a Visualforce page, you also have the option to skip the record type selection page. If selected, any new records you create won't be forwarded to the record type selection page, since it assumes that your Visualforce page is already handling record types.
Optionally, enter any comments to note the reason for making this change.
Button overrides are global throughout Salesforce because overrides control the action behind the button. For example, if you override the New button on opportunities, your replacement action takes effect wherever that action is available.
The Opportunities home page.
Any opportunities related lists on other objects such as accounts.
The Create New drop-down list in the sidebar.
Any browser bookmarks for this Salesforce page.
A standard button (New, Edit, View, Delete, and Clone) overridden with a Visualforce page won’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.