The project MCU target defines the build and debug settings for your target device - compiler settings, memory layout for linker scripts and debug launch configurations.
For application projects, you should always match the selected MCU part for the project to the actual target MCU.
To change your project MCU target type:
select your project in Project Explorer
Right-click on the project and select Properties
Expand C/C++ Build and select MCU Settings
Select the correct target device from the list
In addition, the project target MCU is displayed in the status bar in the bottom right-hand corner of the IDE. The MCU Settings dialog (described above) can be opened by holding down the keyboard Ctrl button and using the mouse to click on the MCU name. For more information, please see the FAQ Status bar shortcuts.
Changing MCU Families
If you change the MCU family (for example from an LPC17xx to an LPC13xx), rather than just the MCU being used from within a single family (for example an LPC1768 to an LPC1754), then you will also need to replace the startup code used by the project. This simplest way to do this is to create a new project with the required target MCU and copy the startup file across.