how do i increase my reaction time!

Basically, you improve just by playing, but if you want to speed things up a bit, you can use Human Benchmark and do KovaaK's exercises like Gridshot or Piano Tiles. If you play Valorant, there's a site called Oki Aim—it's pretty good for a web-based tool. If you play CS or Fortnite, you can use in-game maps. Anyway, this is just what worked for me; it might not result in a massive improvement, but I hope this helps! :D