The spell's description explains it, but I've broken it up into a point form list below of what changes and what stays the same.
Properties that are changed by the spell
These are the things changed by the spell, so use them instead of the weapon's properties.
- Damage - The weapon always deals 1d8 damage, +1 per three caster levels of the caster.
- Spell Effect - It ignores damage reduction. It is subject to spell resistance. If the creature you attack has spell resistance, roll a caster level check against that SR the first time you attack. If you succeed, you can attack that creature with the weapon for the entire duration of the spell without another check. If it fails, the spell is dispelled.
- Force Effect - It can strike incorporeal creatures without the usual miss chance against them.
- Attack Roll - The weapon uses your Base Attack Bonus + Wisdom modifier as it's attack roll.
- Feats - The weapon is not affected by your feats, even if a normal weapon if it's type would be.
- Combat Actions - The weapon is not affected by your combat actions. It can't do special things like trip or set against a charge, even if the normal weapon has a property that makes it good at those things.
- Flanking - The weapon does not get a flanking bonus and does not give someone else a flanking bonus.
- Range - The weapon's maximum range is the spells range, even if it's a ranged weapon that would normally be able to use range increments to shoot farther. Switching targets still requires a move action even for a ranged weapon. For melee weapons, a move action lets it get one attack against the new target, so essentially it can move anywhere within the spell's range when it's moving.
- Vulnerability - A spiritual weapon cannot be attacked or harmed by physical attacks, but dispel magic, disintegrate, a sphere of annihilation, or a rod of cancellation affects it. A spiritual weapon’s AC against touch attacks is 12 (10 + size bonus for Tiny object).
Properties of the Weapon that are used
These are all the things where you use the properties of the weapon itself (whatever weapon that happens to be).
- Threat Range and Critical Multiplier (If it has an improved threat range, you get to use that.)
- Anything else not listed