You can use an extra object ( Let's say an empty ) which will hold a boolean and will change it's property each click .
And just put that object in the UIButton section in the Button object ( In "On Click -> Notify" ) and then chose the function which change the variable for that boolean in the method section that will open gor you ones you drug the new object in it .
In the new object you should put some function like that :
bool Clicked = false;
public void BoolChanger() //make sure to make the function public so you can access it via the inspector !
{
if(Clicked)Clicked = false;
if(!Clicked)Clicked = true;
}