How to update a value with reducer via text input?

Question

I would like to update the value of a variable in the reducer. In the app the user can type the username in a text input field in one screen. The reducer should update this state so that I can output it on a different screen. Right now I'm using useState but I'd like to use the reducer instead. I'm new to coding and I'm really struggling with this so I hope someone can help me.

The reducer:

    import { createSlice } from '@reduxjs/toolkit';

    const userSlice = createSlice({
        name: 'user',
        initialState: " ",
        reducers: {
            setUser: (state = initialState, action) => {
                ???
            },
        }
    });

    export const {setUser} = userSlice.actions;
    export default userSlice.reducer;

Code in the screen with useState:

    const [user, setUser] = useState("");
    const [userName, setUserName] = useState("");   
    const onValueChange = (number) => setUser(number); 
    const onButtonHandler = () => setUserName(user);

       <View style= {globalStyles.containerInput}>
          <TextInput 
             style= {globalStyles.input} 
             maxLength={20} 
             minLegth={10} 
             keyboardType="number-pad"
             value={user}
             onChangeText={onValueChange}
          />
       </View>
       <View style= {globalStyles.containerButton}>
          <Pressable 
             style={globalStyles.button}
             onPress={onButtonHandler}
          >
          </Pressable>
       </View>