I have created a design in Responsive Layout Maker and exported it and tweaked it -- added images, some minor layout changes, etc. Now I see I need to make some changes in the responsive layout maker -- modify, add breakpoints, etc.
Is there a way I can load my file into responsive layout make and make changes, and if not, how would I do this?

You will need to reopen your project and make the needed tweaks then readd the other content since your making changes to the actual layout.

It would also be highly beneficial to you to "not" add your actual content and/or tweak the code (other than for testing to see if it's what you want) until you have your layout fully developed so as to save you a ton of time. If you're tweaking and adding things and then realize you need changes, that's a pretty backwards way to work it. So...

Get your layout done completely first. Get the main layout done, then use the slider in the program to check the viewport sizing, don't do it outside of the program as you're making more work for yourself. That's why it's all built into the program to check all that first.

Once you have the layout complete, then most likely the only changes you may need later might be changes that are only rendered in the CSS or added pages. This type of change is easy to do as when you export your layout with your newly added pages, you can just take those pages, grab the .js files and .css files and that's it. You won't always have to overwrite your current pages then as they most likely won't need any structural changes done if you've completed the layout first for them. Keep it simple