My question pertains to the sensor jammer. The card reads, when defending, you may change the attackers (hit) results to a focus result. The attacker cannot reroll the die with a changed result.

My question is this. If I change a hit result, and the attacker has a focus token, can the attacker change the focus back to a hit?

Yes.

The card doesn't say anything about not being able to modify the results, only that you can't reroll them. Rerolling is a way to modify the dice, but is not the only way.

Yes. The only type of modification that is forbidden by Sensor Jammer is a reroll. A focus token, Poe's ability, Guidance Chips, Palpatine (and a bunch of other abilities) would still allow that eyeball to be modified.

Thanks guys it is greatly appreciated

Remember that after the attacker rolls his dice, and you modify them before he does. And this is where your Sensor Jammer triggers. If the attacker doesn't have a focus token (or similar ability) this is effectively giving him a blank result, because he can't reroll it. It kind of makes it essential to have a focus token when attacking a ship with Sensor Jammer, and that means the attacker has limited action options if he wants to hit you. It's a little expensive at 4 points, but the action limitation effect can sometimes help balance it.