You'll need to have a new front sight staked in the slide.
Front sights have a stud or "tenon" on the bottom.
The sight is inserted into the slide, and a special staking tool is used to stake or rivet the tenon to lock the sight in place.
There are old style, narrow tenons, and new style wide tenon sights.
The wide tenon is as wide as the sight, the narrow is narrower than the sight.
You MUST have the right type for your slide.
Unless you have the special staking tool and know how to do the job right, a new front sight will not stay put.
You can find a local gunsmith who really KNOWS how to do the job correctly, or you could simply ship the slide to Auto Ordnance Thompson and they will install a new sight.