when I use `<< PhysicalConstants`` I get the message like :General::obspkg: PhysicalConstants is outdated... see the Compatibility Guide for more infomation... The question is how to update it? My version is MA9.0.
It can't be updated. The Physical Constants package is obsolete as of 9.0 and is no longer updated. You can ignore that warning message and still use it if you want, but mostly the same functionality is now provided by the Units framework (new in 9.0).
In addition to the warning message you saw, there's a note at the top of the Physical Constants package guide page to this effect. That note is suffixed with an unfortunately barely visible link to the Units framework guide page.