You may change any number of the targets, including all of them or none of them. If, for one of the targets, you can’t choose a new legal target, then it remains unchanged (even if the current target is illegal).

If the targeted spell is modal (that is, it says “Choose one --” or the like), you can’t choose a different mode.

If you cast Redirect 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 Redirect. If you do, that spell will be countered when it tries to resolve because Redirect will have left the stack by then.

Redirect can target any spell, not just an instant or sorcery spell. For example, you could use it to change the target of an Aura spell. However, if the targeted spell has no targets (for example, if it’s an instant or sorcery spell that doesn’t specifically use the word “target,” or if it’s a creature spell), Redirect won’t have any effect on it.