Use positive-diopter ("reading" glasses). If your eyesight is normal, you might need to get an optometrist to cut you a mild positive prescription, as the lenses sold over-the-counter in US drugstores are generally stronger than a young person would use in this context... But you can try one of the milder ones and see how it fits your working distance.

Such lenses will "push out" the focal distance of your screen, so it is optically closer to infinity. If you already wear negative-diopter glasses, for nearsightedness, you can get a milder negative prescription for close work or check out the new "computer" lenses that are multi-focal and give you different zones for nearer, farther, and in-between.

Your eyes "work" to focus more closely-in, via contraction of the relevant muscles to thicken the lens. So prolonged close-work, such as with a computer, will cause fatigue and eye strain. There is also decent evidence, though it is debated, that prolonged exposure results in a trophic response and acquisition of myopia (nearsightedness). I've seen this empirically multiple times.

Wearing glasses for nearsightedness while you're doing close work is a double-whammy. They make things look even closer. At the minimum, don't wear these glasses if you don't need them at the computer.

Hope this helps!

BTW if you don't entirely trust ergonomic advice from a random dude on the internet, your local optometrist can help. There is a type called (in the US) a "behavioral optometrist"; these folks generally are more focused on optimizing for occupational needs like this, vision therapy etc.

PS. Hey, just realized this is a physics audience! You guys can figure this all out yourselves with the thin lens equation... :)