How to straighten a bent aluminum extension ladder?

Without seeing the degree of damage this is hard to say but, once bent aluminum beams like your ladder is constructed of will have lost a significant part of their strength. Any attempt to straighten it can result in cracking and further weakening.

Gently apply weight at the bend point with the ladder supported 2-4’ away from the bend until it is straight as possible. Look over the bent area for any signs of the normally smooth metal surface beginning to show cracks or ‘wrinkling’. It will be there. In truth aluminum will separate before returning to its original shape every time.

Decide if you want to risk falling from that height. Either cut it short or reinforce it with aluminum or stainless steel as any other metal will cause corrosion and further weakening. Any repair should be long enough to bridge into clean metal by a foot or so each side of the bend and be well attached to the wider portion of the beam covering its full width.

OSHA will not allow your ladder on any job site.

Proceed with caution!