how do I unplug the round power cord from the back of my IMac?

• Switch off the mains
• For extra safty take the plug out of mains
• Remove the powercord perpendicular to the iMac.
• Moderate constant force
• Don't use tools as correctly mentioned by seventy one

Best Wishes,

AK

Turn off at the mains then pull gently, jiggling it about as you do so. It can be a bit tight but it does come out.

Don't use tools on it, especially pliers: they will damage the cable.