Use a script command and type that in. Replace weaponId / armorId with the id of the weapon and you want to reset.
Viewing post in Synthesize Equipment Plugin - RPG Maker MZ comments
If you meant a dedicated UI to reset weapons, then no, I did not add such a feature as I've stayed true to the reference system. Perhaps if demand picks up, then I may add it in the future, or you may also commission me here to customize it according to your preference.
For now, you will have to make do with that script call, and a combination of the Input Number command to get the desired weapon/armor to reset:
1. Add an NPC that resets weapons/armors
2. Add a Show Choice command to determine if the player wants to reset weapons or armors
3. Add an input Number command to get the weapon id or armor id to reset, that id would then be stored in a variable
4. Call the below script call. Replace accordingly.
$gameParty._synthBonusForWeapon[$gameVariables.value(variableId)] = [0,0,0,0,0,0,0,0,0]; or $gameParty._synthBonusForArmor[$gameVariables.value(variableId)] = [0,0,0,0,0,0,0,0,0];