How to override a standard button or link for an object in Salesforce

In Salesforce, you can override the standard button or link of an object to implement our particular logical process, such as determining the state of a field in an object before deleting an object Or you want to assign values to some field at the time of new object.

1): as shown in the following figure, it shows all the button,link and action in an object, then the red box is the standard

2): Here we take the new button as an example to show, click on the new map in front of the corresponding edit operation, will appear below the icon, the default state selected is no Override, here we want to select is Visualforce Page, You can select the corresponding page in the Drop-down list on the right