You need to be clear what your aim is and whose method you are employing. If you are already under the guidance of a teacher then I recommend that you follow the instructions precisely for at least a year. If you are not following a particular teacher's instruction then I suggest that you follow only one particular method for at least a year to give it a trial.
There are different variations of anapana-sati that are propagatd by different teachers. The two main strands of anapana-sati relate to its intended effect of cultivating samatha or vipassana.
I think most teachers will recommend that you maintain your awareness of the object and if you find yourself being distracted by mental contents, to return to the breath.
In the absence of a teacher, I recommend you have a look at some foundation literature.
The section on anapana in the Visuddhimagga is excellent and I highly recommend it. Search for 'anapana' and it should direct you to the section.http://www.accesstoinsight.org/lib/auth ... on2011.pdf
Likewise, the Manual of Respiration by Ledi Sayadaw:http://www.aimwell.org/Books/Ledi/Anapa ... asati.html
If you are just beginning in your practice and unless you have developed any of the jhanas then stay with the first tetrad.
All the best,