How to change an icon image?

You can copy an icon from one item in the Finder to another item in the Finder by using the Get Info (cmd-I) window. The icon that appears in the Get Info window can be selected, then copy it (cmd-C), select that icon in another Get Info window and paste it (cmd-V) when you first select the icon you want to replace.

when the icon is selected:

Please help us help you—right now your question is far too general. What PowerMac model is involved and what operating system version is it running.

Those are important for us to respond in other than useless generalities in a setting where we cannot see, touch, nor access your device. We must rely on you for all details.

Also this section is for posting questions about ancient Mac desktops make between 1998 and 2005. Is yours that old??