How to Upgrade very big application from rails 3.2.0 to rails 4.2.1

I am working on very big application which is build in rails 3.2.0. Now i want to upgrade that to rails 4.2.1.

After updating all gems (using bundle update).. I am facing lot of issues like scope is not callable ,some gems compatible error and devise - form is not working.

What is the best way to upgrade rails application??