Changing the resolution depends on the tool you are using.

When you open an image on a Mac in the program Preview, for example, in the menu there is a "Tools" item where you can select "Adjust Size...":

If you click on "Adjust Size...", then you get an option to change the resolution:

Often there are videos on YouTube where someone will have done a screencast of how to do things like this, in your favorite program for opening and editing images on your computer.