If all you want to do is to open one or more files that are on your back up drive, the procedure is exactly the same as opening a file on your main drive, viz. double click on the drive icon, then navigate through the folders to the file you need. Then double click to open it. Or you can drag and drop the file to your desk top (or anywhere else).
If however, by "access the files I've backed up" you mean replace all or (some of) the original files with the versions on your back up drive, the specific procedure depends on the back up program you used. Typically, the process is called "Restore", and what you can do and how you do it, depends on your back up program.