To save your presentation, choose the command File | Save 
.
Tip: This command can also be found in the Quick access toolbar below the ribbon.
When you choose the command Save for the first time in a newly created presentation, the dialog box Save as opens. There, type any name for the presentation in the box File name. You can also switch to another folder if you want to save the file somewhere else.
Let's name the presentation Practice. Thus, enter "Practice" in the box File name, and click on Save to confirm your entry. Presentations will save the file under the specified name and will automatically append the extension .prdx. Thus, the complete file name in this example is Practice.prdx.
The next time you choose the command Save, the program will no longer display this dialog box because the presentation now has a name. It will thus be saved immediately under its name.