You can change the icon of most items in the system by getting information on them (select and press command-i) and then dragging an image of choice to the small icon at the top of the information window.
If you would like to use an image that is currently the icon for another application or document, then get information on that item, click the icon at the top of the information window to select it, and then press Command-C to copy it. Then get information on your target item and similarly click its icon, followed by pressing Command-V to paste it.
What is it you are really trying to do?
A package file is normally an installer of some sort. Is it a programme you created and want to have the programme icon show up instead of the package? I don't know, but you may want to seek help on the developer websites.
Did you download some software and it shows as a package instead of a program? Double-click to run the installer and it will put the programme into the Applications folder.
Or, do you just want to change the icon for its own sake? See Topher's post.