If you're referring to the frequency of PWM as the title says, you can use the various PWM modes to generate PWM as required. Let's say you're going to use Timer 1 for PWM. Make sure that the TOP register is ICR1. Then, the value of ICR1 will give the required frequency and duty cycle will be controlled by OCR1. You can use Fast PWM, Phase Correct PWM or Phase and Frequency Correct PWM with ICR1 as TOP. These are modes 8, 10 and 14 of the possible PWM modes. You can find this in page 109 of the datasheet. Download the datasheet and go through the modes of operation parts of the timer sections to understand the different modes of operation.
Hope this helps.
Tahmid.