Under the settings for your classifier, you can use the Classifier Algorithm setting to toggle the algorithm that is powering the model Currently you have two options:
Multinomial Naive Bayes or MNB, is a very simple and fast algorithm that has very good performance in most cases. Read more in wikipedia.
Support Vector Machines or SVM, is a more complex algorithm, slightly slower than Naive Bayes but delivers a higher accuracy in general. Read more in wikipedia.
Try different algorithms to see what affect it has on your classifier performance.