Point your mouse at the name of the file you want to change. Click once and do not move the mouse. A box will appear around the name of the file with the
old name highlighted. Type in the new name of the file, and then click the mouse pointer anywhere outside the box. The editing box will disappear, and the
file will be renamed.
I assume you are a new Mac user as this is a very basic question (although the answer is not obvious). Pressing the HELP key on your keyboard, or selecting
Mac OS Help from the HELP menu in the Finder brings up the help screen which will answer questions like this. There are also many good iMac books
which not only explain the operating system but also show you how to use the software which is bundled with most Apple computers.
Have fun. I hope this helps. I am posting this reply, as well as emailing it to you, so you won’t get 100 answers to your question... Marc