HI how can I cancel/terminate/give up green card obtained one year ago? What department and form I shall use to proceed?

You must return to your home country and contact the US consulate. After processing, you will be issued a receipt. Submit this receipt when you apply for visitor's visa, you will be granted a multiple entry visitor's visa for 10 years.

HI how can I cancel/terminate/give up greencard obtained one year ago? What department and form I shall use to proceed?

Very unusual form, very rare, I-407, abandonment of lawful permanent residence. :( Are you sure you want to relinquish? You can file with the USCIS or AMCON abroad. Good luck, I hope this helps.