How do firms go about deleveraging if they have high debt? how can they retire their bonds and reduce their D/E ratio if it's already issued?

Just look at how Oxley is doing it now. They sell assets at HOPEFULLY a higher price to repay their debt.

Oxley strategy seem to be borrow at a low and buy assets, then in return sell the assets at a much higher price in future. Earning the spread.

It sound like trading.