While installing my car stereo system myself.

While installing my car stereo system myself, or more like after finishing installing it, I put the battery terminals back on, and noticed when I tried to remove negative terminal again, my socket set striped the screw.(Screw in type on the side of battery, not top) Anyone know a way I can get screw off, so I can replace with new one? Its stuck on there now. Also, if I need to make a repair before this is fixed, and need to remove terminals again, whats the risk I face if I remove positive terminal 1st, due to negative being striped? Although, I feel it may be best to get the negative terminal problem fixed 1st.(Yes I am new to this, and learning) Help would greatly be appreciated. Thanks to whomever will reply on this post. Also, in the engine bay, what color is the horn relay? When I purchased car, they where hangling, and not in place. Repair manual does not give color, only placement. Furturemore, my car did not come with its owner's manual, nor repair manual. So, I bought one. Its from Chilton.