Just cut off everything above the top two arms. That will cause strong growth from the area of the cut. Either train that over to replace the top arms or cut it off every 6 weeks.

You can also cut off or train any strong uprights off the four arms of the espalier.

You could also add another set of arms higher up.

If you just want more fruit cut out those strongest limbs up high and save everything else..

Maintain shape by summer pruning anytime growth gets out of bounds.