direct replacement. The rear sight should be easy enough to drift out with a brass punch, but I don't know how to do the front...
If it's a stock type slide, the front sight is staked into place.
You drill out the old sight and stake the new one in place.
There's a special modified pair of vise grips for the job as well as other special tools.
The other way (which I feel is nicer too) is to mill a dovetail in the slide and install a dovetailed front sight.
That's what I have done when replacing front sights.