You can't save a part in an older version after it's been saved in the newest version. For example if you work on a part and save it in NX6 you can no longer open and save that part in NX5. There is no backwards compatibility in the NX products.
You can however save the solid/surface model (without features or history) portion of a model (the Parasolid data) so that it can be opened and used in an older version of NX. Just go to:
File -> Export -> Parasolid...
And then from the 'Version' drop-down list, select the version of NX that you wish to move the selected body(s)from your current file to. Once that that has been done, you go to that version of NX and using the File Open command change the 'File of Type' option to Parasolid and open the selected Parasolid file as an NX file.
John R. Baker, P.E.
Product 'Evangelist'
NX Design
Siemens PLM Software Inc.
Cypress, CA
If you save files as parasolid to use in older versions of NX during the export observe that there is an opporunity to select a version of the parasolid export format suitable for your older version of NX. The latest by default would usually be too new for any older version of NX. This is true of all versions up to NX-5 and might persumably be equally the case for NX-6 although I haven't been able to check at the time of posting this.
This is a prime example of something that should be able to be answered by doing a search, as the question has been asked innumerable times, and the related threads offer good explanations as to why it is not possible to save backwards.
FAQ anyone?
When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - [small]Thomas Jefferson [/small]