You don’t choose the new target for the spell until Ricochet Trap resolves. You must change the target if possible. However, you can’t change the target to an illegal target. If there are no legal targets to choose from, the target isn’t changed. It doesn’t matter if the original target of that spell has somehow become illegal itself.

If you cast Ricochet Trap for , you don’t have to target the blue spell an opponent cast.

If you cast Ricochet Trap targeting a spell that targets a spell on the stack (like Cancel does, for example), you can’t change that spell’s target to itself. You can, however, change that spell’s target to Ricochet Trap. If you do, that spell won’t resolve when it tries to resolve because Ricochet Trap will have left the stack by then.

If a spell targets multiple things, you can’t target it with Ricochet Trap, even if all but one of those targets have become illegal.

If a spell targets the same player or object multiple times, you can’t target it with Ricochet Trap.

Ricochet Trap targets only the spell whose target will be changed. It doesn’t directly affect the original target of that spell or the new target of that spell.