how to increase QA score from 49 to 51?

Increasing the raw score from 49 to 51 is difficult. You make the same amount of mistakes in both those cases, but how frequently you make mistakes decides whether you score a 49 or a 51. Say you make 6 mistakes. In the case of 51, those 6 mistakes would be separated by right answers. Say, q nos 5,11,18,23,30,36. In the case of 49, those 6 mistakes would consist of successive errors. Say, q nos 21,22,23,29,31,33. GMAT penalizes successive errors.

You can beat that by getting all 37 right, but its difficult to do so. Practice more and give yourself the best chance. Though, 2 points in quant would not make a big difference for your score and the 49 and 51 don't make any difference for the adcoms.