'Dev' projects are folder based. Everything for the project goes in the same folder.
I would start be creating a folder and downloading the library ".cpp" and ".h" files that you need directly from Github.
If you look in the libraries list in 'Build', there is a Github icon for each library that will take you to the right place.
Then start a new file in Dev, and copy and paste your own code from Build. Name your own files '.ino' or '.c' and they will be included in the compile.
Since everything is in the same folder, remember to remove any paths that Build may have added to your include statements,
Remember to save every time before compiling; there is no auto-save before compile, and if you do not save, it will compile your 'old' code from the last save.
To backup, just go to your regular file manager and copy the entire folder to a new folder. I do that whenever I make enough changes that it would make me angry to lose them by mistake.