how do you change yahoo back to google on safari on a mac

See the Safari User Guide for Mac entry for this technique. In essence you open a new browser window where the smart address bar is blank. Click on the magnifying glass, and the list of selectable search engines will appear. Click Google.

I have been doing this but it still appears the yahoo website. I don't know what else to do.

You may have the Yahoo redirect malware installed.

See: https://malwaretips.com/blogs/yahoo-toolbar-removal/#mac