how to study again,

Have you received your candidate performance report? That can help guide you a lot.

The second page (backside) of the CPR provides information regarding your performance within the 8 NCLEX Test Plan content areas of the exam. The content areas are grouped by the quality of your performance (ability): "Below the Passing Standard," "Near the Passing Standard" or "Above the Passing Standard." Next to each content area (called "Client Subneed") is a description of the content area, its percentage of the test, and a list of topics related to the content area. These descriptions can be used to determine in which areas of the Test Plan you have shown the greatest weakness and as a guide to prepare you to retake the exam. You should concentrate first in those areas listed under "Below the Passing Standard" and work up to those areas listed in "Near the Passing Standard." Furthermore, even though your ability for a given content area may indicate that it is above the passing standard, these areas should be studied as well in order to maintain proficiency.

More information can be found here.