There are a whole bunch of different weather apps, one of which you have. The answer for how to operate them varies with the different apps. If you can identify which app you have, it would likely give you more precise answers.
I use "NOAA Weather Free" from Granite Apps and think it's pretty nicely done. It has a bullseye sort of button at the top of the screen to reset the current position to wherever I am, and a little side menu on the lefthand side to store a few locations (There's a paid version available that removes ads and lets one store any number of locations.)