If you have OS3.9 (perhaps OS3.5, too) you can click once at the old icon of Asterix or its default icon if there is no real icon assigned to the executable yet, select "information" from the "Icons" menu and then you drag 'n' drop the new icon into the icon box of the information window. After that, press "save" and you are ready.
If you have an older OS, you can use the CLI:
copy source_path/asterix.info destination_path/asterix.info
Perhaps there are tools with a GUI that does this job on Aminet. Try SwazInfo.
