How can I make the "+" change to "-" when pressed?

Question

JavaScript

The site has a horizontal menu, in the mobile version I made it to expand the categories "+", I can't figure out how to change it to "-" when I click on it,
so it doesn't want to work

.fa.fa-minus{
display: none;
}
.fa.fa-plus{
  display: block;
}


.dropdown .open .fa.fa-minus  {
display: block;
}

.dropdown .open .fa.fa-minus  {
display: none;
}

Answer 1

you have the same selector described twice -.dropdown .open .fa.fa-minus

Answer 2

1) The selector itself is incorrectly specified, you need to write

.dropdown.open .fa.fa-minus  {
display: none;
}

those. dropdown and open without a space
2) It won’t work for you even after the fix, because you put the icon not inside the dropdown, but in the neighborhood, and before it, and not after, you can’t even pick up the selector, or put the icon inside and then the first one will work option, or after the dropdown block and then the selector will look like this
dropdown.open + .fa.fa-plus {}