There is no way to change the model of your cRIO target in the project. You will have to add a new cRIO target:
1. Right-click
Project >>
New >>
Targets and Devices ...
2. This will bring up a dialog. Select
New target or device and pick the new cRIO type from the list
Real-Time CompactRIO.

3. Configure the new RT CompactRIO Target IP address by right-clicking >>
Properties. You can find your cRIO's IP address from NI Measurement & Automation Explorer (NI MAX).
4. [Only if using FPGA Module] Right-click the newly-created chassis >>
New >>
FPGA Target.
5. Drag all VIs, modules, FIFOs, etc. into the newly-created RT target.
Additional Information
It is better to add a new RT target using
New target or device instead of
Existing target or device if you have multiple cRIOs of the same type you have to deploy to. If you add using
Existing target or device, the RT target will be named according to the serial number of a specific cRIO, reducing interchangeability.
File >> Save As >> Duplicate .lvproj file and contents can be used to create backups and different versions of your LabVIEW Real-Time project.