Open the Project Properties Window
To change the MCU used in a project the Project Properties Window must be opened.
Select the new MCU to use
Near the top of the Project Properties Window you will see two pull-down menu boxes. One for "Family" and one for "Device". The device box displays the MCU curently being used. Changing the part number is the Device menu will instruct MPLABX to make and image for that particular MCU the next time the project is Built
Since Microchip makes several hundred different MCUs MPLABX provide the Family menu. Selecting the family to use before selecting the device may reduce the time needed to change the MCU designation.
When a tool has been selected it will appear within a blue box in the Hardware Window
Confirm the Selection
After you have selected the Hardware tool to use select "Apply" then "OK".