What are the complications if i don't remove mirena (levonorgestrel) iud after the 5th anniversary? if i won't replace it with new one - can it be left inside awhile?

No guarantees: That it will still be effective at preventing pregnancy; and increased risk for ectopic pregnancy if conception occurs, and risks to baby if pregnancy in the uterus occurs with an iud in place; also: risks of it becoming 'scarred' in place, and being very difficult to remove; as always, risks of infections (abscesses in the fallopian tubes); risks of it poking into the wall of the uterus with pain.