These are the conditions I have to put agreement start date can't be earlier than today 

I have created the custom object.My requiremenet is i need to change the vendor name . So when I change the vendor name i need to get a popup box with check box yes or No to select.

In my custom object ,I have edit option to change the Vendor name but when I save I need to get the pop up box with check box.So i need to overide this save option but in the object under button& link secton I m not able  to  see save button .so  how I can override the save  button  
for creating the popup box please suggest .Thanks in advance .