Where do you set field spells(Yugioh)?

To the left of the monster cards zone which are 5 cards above the spell and trap card zone which is to the left of your main deck which is below your graveyard.

you have to destroy the old one and place the new one in its place but ive never heard of a field spell the needed to be set to activate its effect

All spell cards are set in the spell and trap card zone, then if you activate a set field card spell, it destroys any field spell currently active and moves to your field spell slot. So you are correct to assume that field spell are set in the regular spell and trap card zone, that said, if desired, you CAN actually "set" them in your field card spell zone.

Field Spell Cards have the advantage of being able to change the entire state of play, for both you and your opponents side of the field. Most cards center around boosting ATK, DEF or both, for cards with Specific Attributes, or Types. They are Spell Speed 1. Field Spell Cards must be placed on the Field Spell Card Zone, and there can only be one in play (face-up) at any one time. If one is already active, and another one is activated, the previous Field Spell Card is destroyed. You may Set a Field Spell Card at any time during your main phase; any card that currently occupies that Field Spell Card Zone is destroyed. If you Set a Field Spell Card, and your opponent already has one active, the opponent's Field Spell Card is neither destroyed nor negated. Setting a Field Spell Card does not start a chain. Playing or flipping a set Field Spell Card does start a chain. The effects of Field Spell Cards do not start a Chain.

Field Spell Cards are placed in the field spell zone not where other traps and spells go. When a new field spell card is played the old one is destroyed.