There is no fixed button to disable the DPU on a mouse. If you are using a Logitech Mouse, you should download the Logitech G Hub software; for a mouse of any other brand, download the FinalMouse Software. After you have installed the software, click on the mouse icon and click on the "+" symbol that can be seen on the left-hand side of the screen. Once you have selected the Assignment Options setting, you will be shown a 3D design of your mouse with a graphical overview of what each button present in the mouse does. Search for the DPI button on the 3D design and click on it. Clicking on the mouse should provide you with a disable option.