Can you respond to your own spell on the stack by activating an ability that changes its targets?

When you cast a spell, it is placed on the stack and then you — the active player — receive priority (CR 117.3b). While you hold priority, you may activate abilities or cast spells before your own spell resolves. This is standard stack behavior and applies to everyone, including the spell's caster.

Abilities that change a spell's target (such as those granted by cards like Willbender, or activated abilities on equipment or auras) follow the targeting rules under CR 114 and 115. If such an ability lets you pick a new legal target for a spell you control on the stack, you may absolutely activate it in response to your own spell while it is waiting to resolve.

One important distinction: simply changing a target is not the same as redirecting it. The ability doing the work must explicitly allow target changes (e.g., "change the target of target spell"). You cannot arbitrarily retarget a spell without such an effect (CR 115.7).

Concrete example: You cast Lightning Bolt targeting your opponent's creature. Before passing priority, you activate an ability (say, from a card that reads "Change the target of target instant or sorcery spell to another legal target") and redirect Bolt to a different creature. This is perfectly legal — you responded to your own spell while holding priority.

Keep in mind that once you pass priority and your opponent also passes, the spell resolves. You must act before that window closes (CR 117.4).