Under image on the menu choose image size and select the size you want. going smaller is great going larger will cause pixilation.
You should have a pull down menu that will allow you to select the size in inches instead of pixels. You can type in one size and it will complement it with the other size.
That way you can directly size the image to fit without calculating.
Then you need to check the dpi again. When you resize it sometimes tries to change the dpi when you make the size adjustment.
It has been a while since I used photoshop. Had several versions of Adobe Suites.
All I use now is GIMP. It’s free and opensource and will do anything I ever need to do. And it takes a lot less resources to run.