You can try to shift your focus to only present. You can also read about GROUNDING techniques, it worked for me.
Don’t focus on unnecessary things and just shift your focus to today
Some of the major reasons for overthinking is may be you are thinking more about your past or tensed about future. All you can try is things which you like to do this can be helpful for you to focus in your present.
you can just focus on one sound like tik tok of the clock or any other sound. it will distract you from thinking anything.