How do I undeclare US Permanent Residency

[fast_freestyler]

Hi I have a 10 year expired permanent residency from the united states ive been in canada for 3 years and i want to undeclare my us permanent residency ive contacted the us consolate in Toronto and there is no information about undeclaring american permanent residency what forms would i need and where do i need to go should i show up at the office with my pictures for the application form instead of having to take it home and reschedule another appointment