How to Switch from Intel HD 4400 to NVidia Geforce in my Lenovo Ideapad U430p?

Make sure your computer is plugged into power. Sometimes to use a dedicated graphics card the computer has to be plugged into power. It should use it be default after that.

The nvidia graphics should kick in as soon as you try to run any 3D accelerated program or games etc.
The intel integrated graphics are used for routine stuff to save power.

If it's not switching over when you run a game, make sure you are using the Lenovo video driver for that model.

The makers drivers have extra facilities to control the changeover between the two graphics systems & using a generic intel or nvidia driver can disable that facility.

Some can have drivers updated, but it may need custom installs or manually changing files.
Have a look in the lenovo section of notebookreview.com, that's the best place for technical info such as this.

NBR Lenovo forum: http://forum.notebookreview.com/lenovo/