In reply to aliabosaleh:
Did you run 'grep -r m_sequencer ovm-2.1.2/src'?
If you did, you will find m_sequencer defined in ovm_sequence_item.svh
This may seem confusing, except when you follow ovm_sequence is extended from ovm_sequence_base, which is extended from ovm_sequence_item.
I would recommend NOT changing the arbitration method from a sequence. The arbitration method should be specified at the test level. You should therefor change it in the test.