MCP19035 buck converter controller
For each particular chip the frequency is not selectable by any means. You have to order proper part, and design components based on your selection.

The frequency is selectable based on part order suffix,  MCP19035T-Axxxx is for 300 kHz, and  MCP19035T-Bxxxx is for 600 kHz, see page 41 of the datasheet.