The stems can't be straightened at this point, though new ones can be staked if they start to develop a lean. The plant may not be getting enough light, which caused the initial bend in the stems and continues to influence its aberrant growth. Dracaena prefer moderate to bright-indirect light, which would either be in the direct path of light from a sunny window or up against a window that doesn't receive much if any direct sun. Several feet away from a window reduces the light a plant receives significantly, even though it doesn't look too different to our eyes.

They do respond well to trimming, so if you wanted to drastically cut back the long stems and let new growth replace them, this new growth, being more pliable, could be staked to prevent the same thing from happening to them. You can do this one stem at a time to see how the plant responds and so it doesn't look denuded for too long. It may take a few weeks for new growth to appear after pruning. Rotating the plant about a quarter-turn every week or so will help balance-out the light levels the plant receives, reducing the likelihood it will lean as much in the future.

Miri