Remove road wheel
Mark position of washer on inner end of radius arm as this adjusts the tracking
Lubricate all bolts to be undone
Undo outer securing bolt.
Free off outer securing bolt.
Using suitable drift tap out outer securing bolt.
Undo inner securing nut while holding the bolt in position.
Fit new radius arm, making sure that the inner bolt does not rotate, fit the road wheel then lower the car to the ground and do up the final turn of the bolts so the bushes are in the correct position with the car on the ground.
Get rear suspension geometry checked.