Updating a progress bar

I am new to using Visual C++. I used a Control box to create the progress bar in one file. In another file, I want to update the same progress bar so that it looks like it is downloading. How do I do that?
For example:
one file, Progress.cpp:
Creates progress bar.

Update.Cpp
Need to update progress bar in Progress.cpp:

send the HWND of the progress bar to the update function, or send the progress parent's window and the progress identifyer, and get it's handler with 'GetDlgItem()'; then simply 'SendMessage()' with that progress handler and with the progress predefined messages to update it.
niara

Thanks for the assist. Is there an example of that? I never used those type before. All i have for the progress bar is a member variable that I named m_pBar and the dialog box, labeled IDD_Progress, where the progress bar is located.