Locate your file and right click (Alt click) on the icon. This will bring you a menu with a number of actions on it. Click on Get Info
You will be presented with a window that looks like the image below. Here is all the information about the file you selected, including where it is located on the Mac, the size of it and much more. What I am interested in is the section in the red box.
The Open with: section shows the current app that this file type opens with, and the clicking on it reveals a drop-down that hosts the different application types you can choose. Click on it to reveal the drop-down
You will now be presented with something similar to the image above - a list of all the applications that can open this file type. Select which one you wish to be the new default application.
Tip: If your application is not in this list, you can select Other at the bottom of the list to browse through all your applications on your Mac
Press Continue in the popup dialogue and you are all set!
We probably need a little better explanation on what you mean by "default photo software".
LR cannot view or edit any picture that has not been imported into the LR catalog first.
You can configure LR to be the default program to use when a memory card is inserted to the memory slot or card reader so that your new images can be copied to your hard drive and imported into the LR catalog.
You have answered my question. Thank you