“Flashback [cost]” means “You may cast this card from your graveyard by paying [cost] rather than paying its mana cost” and “If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack.”

You must still follow any timing restrictions and permissions, including those based on the card’s type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you’re paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.

You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.

If a card with flashback is put into your graveyard during your turn, you can cast it if it’s legal to do so before any other player can take any actions.

You choose the words to change on resolution.

If you change the text of a spell which is to become a permanent, the permanent will retain the text change until the effect wears off.

Alters all occurrences of the chosen word in the text box and the type line of the given card.

Can target a card with no appropriate words on it, or even one with no words at all.

It can’t change a word to the same word. It must be a different word.

It only changes what is printed on the card (or set on a token when it was created or set by a copy effect). It will not change any effects that are on the permanent.

Changing the text of a spell will not allow you to change the targets of the spell because the targets were chosen when the spell was cast. The text change will (probably) cause it to be countered since the targets will be illegal.