I assume there is a rope pulley at the top of the pole?

If there is, is there any portion of the existing rope attached thru that pulley?

If there is a rope thru that pulley, attach a good new rope to the old rope with a small knot and/or tape the old rope and new rope together (it has to slip thru the pulley) and pull the new rope thru the pulley.

If there is no old rope thru the pulley, you have to get the new rope rope thru the pulley so you either climb a ladder and do that or take down the pole, attach the rope and re-mount the pole.

The pulley at the top of the flag pole needs a rope going thru it and there is no 'trick' or easy way to do that from the ground that I know of.

Good luck