If you have a Windows Mobile 5.0 phone, you cannot update to Windows Mobile 6.5. Microsoft works in partnership with phone manufacturers (for example, Samsung and HTC) to provide updates, but it’s ultimately the decision of the manufacturer whether or not to make version updates for specific phones available to the public.
Although you can’t update from Windows Mobile 5.0 to Windows Mobile 6.5, you may be able to update to Windows Mobile 6.0 or Windows Mobile 6.1, depending on your current phone and carrier. If such an update is available for your specific phone, the manufacturer or mobile operator will provide it. Be sure to check their web site. If the phone cannot be updated, either because of hardware limitations or because the manufacturer is discontinuing your model, that information should also be available on their web site.
To find out which version of Windows Mobile your phone runs, see
Identifying your Windows Mobile device.