front door threshold replacement
If your existing threshold is structurally sound without cracks or water damage, there is no need to replace it.You can sand it down and then coat it with a marine varnish or some other outdoor sealer/stain. 
If your threshold is damaged in any way, then it should be replaced to prevent tripping. The screws that hold the threshold down are usually visible from the top. Remove them and pry up the threshold. Take the removed threshold to you home store and get a replacement close to your original one. You could also take measurements before removing the threshold and do your shopping before removing it. Seal and attach your new threshold following the direction that came with it.