Theres 2 ways
One is to remove the dash, theres a guide
here
The other one is to do what i did..
You can twist the tweeter covers so they come off then you can force the old tweeter out with a screw driver and feed a wire down through the dash with a coat hanger
I managed the above with a bit of difficulty as i really didnt want to remove the dash unless i had to but its all sorted now
Re-wiring the tweeters with cross overs is another story!