Open the Character menu, pick the character, go to Weapons , then tap or click Switch and confirm the new weapon. In Genshin, you can also do it from Inventory by opening the Weapons tab, selecting a weapon, then using Fast Equip to assign it to the right character.

In the swimming event

If you mean the event’s temporary combat mode, the weapon swap usually follows the same path unless the event locks your loadout. Try opening the character screen during the event, since that is where Genshin normally lets you change equipment.

Quick steps

  1. Open the pause/menu screen.
  1. Choose Character.
  1. Select the character you want.
  1. Open Weapons.
  1. Pick the weapon and press Switch , then Confirm.

If it still won’t change

Some event stages restrict gear changes once a run has started, so back out to the event lobby and change it there first. If you are on mobile, the same flow applies through the on-screen menu instead of keyboard buttons.

TL;DR: go to Character > Weapons > Switch > Confirm.