I want to update my iPhone contacts "from" my iMac computer, not the reverse, and using my iPhone cable connected to my iMac.; how do I do this?

The latest version of iTunes no longer provides a mechanism to do that. You can, however, export your Mac contacts to a vCard, erase the contacts on your phone, then import the vCard to your phone. Open Contacts (Address Book) on your Mac, select the All Contacts group on the left sidebar, click a single contact to the right and press Command-A to select them all. Then go to File>Export>Export vCard and save this on your destop. To erase the contacts on your phone, download the free app My Contacts Backup to your phone, open the app and tap the gear icon on the bottom right. In the window that opens, scroll to the bottom and tap Remove All Contacts. Next, email the vCard you saved earlier to yourself, open it on your phone and tap the attachment to import the contacts from your Mac to your phone.

You're welcome.

I want to do the exact opposite! I want to keep the contacts on my phone and not have them sync when I connect to my iMac. Any ideas?

If you hook up to the newest iTunes, they shouldn't sync. Also don't turn on Contacts syncing in iCloud.