B-Wing; Ten Numb, et al: what does it mean to spend a stress token?

You remove it from your ship.

Whilst it is a little vague in the rules, you must have focus results to convert to spend the stress.

Paying Costs

A ship can pay a cost for an effect only if the effect can be resolved.

• For example, GNK “Gonk” Droid’s ability says “Action: Spend 1 (charge) to recover 1 shield.” The ship cannot spend the charge if it has no inactive shields.

• Replacement effects can replace a cost that a ship would pay for an effect. If a cost is replaced in this way, the effect is still resolved.

Then focus rules:

Additionally: • A ship cannot spend a focus token to change (eye) results to (hit) or (evade) results if it does not have any (eye) results.

RR pg 20

Quote

When a ship is instructed to spend a token or there is an instruction to remove a token from a ship, a token of that type is returned from that ship to the supply.

Basically, just replace Focus with Stress from the rules for Ten. sooo it would read

Quote

While a stressed Ten Numb performs an attack, during the Modify Attack Dice step, he may spend a stress token to change all of his [focus] results to [hit] results.

While a stressed Ten Numb defends, during the Modify Defense Dice step, he may spend a stress token to change all of his [focus] results to [evade] results.