[crtl] S = Save
Save As lets you change the name of the file and the path to save it to.
Save Target is a right-click function in some browsers to allow you to save the item you have right-clicked on to your disk.
All of them overwrite. Save As and Save Target allow you to change the file name so you can preserve the original copy.
Someone with more experience may know if there is a way to merge files. The only way I know is, for example, to load the first one into a word processor, and then use the Insert File (or equivalent, depending on your program) to add the contents of the second file into the first.