toggle classname according to click

If you want to set an "active" class to the selected item that how I would do it:

const [months, setMonths] = useState('January')

const handleChange = (event) => {
   setMonths(event.target.name)
}

return (
<div className="first6-months"> 
   {Data[5].firstsixmonths.map((item, key) => ( 
        <p className={item === month ? "active" : ""}
           onClick={handleChange}
           name={item}> 
         {item} 
         </p> 
    ))} 
</div>
)

1. When looping output, always put a key prop on your element

{Data[5].firstsixmonths.map((item, key) => ( <p key={item} onClick={() => handleClick(key)} > {item} </p> ))}

2. Use some bit of state to track which month is active

const [activeItem, setActiveItem] = useState(0);
const handleClick = (key) = setActiveItem(key);

3. Then set your className based upon active

import classnames from `classnames`;
//...
<p className={classnames({
  active: item === Data[5].firstsixmonths[activeItem]
})} onClick={() => handleClick(key)}>{item}</p>

This is the basics of what you need, but you'll probably need to play with it a little.