Avoid simultaneous modification of an object

I'm building a web app, and so far i have build the CRUD for several database tables. Now, lets suppose that 2 users are using the app and both of them modify the same object but they are not aware of that. So the first user that finish the modification of the object and expect certain state of that object but then the second user finish his modification and the object now will have the state that enter the second user. How do i prevent this. Thanks