When you press the Save button, located at the far left of the toolbar, you are given the choice of either saving the current presentation by pressing the Save button, or saving the presentation to a different location. You can perform the latter by pressing the Save As button.
On pressing the Save As button, a new window will appear letting you save the file with a new name or to a new location. To save the file with a new name, tap in the Filename field and change the filename to the appropriate name. Note that you do not need to include a file extension. To save the file to a new location, tap the location field. Another window will appear allowing you to choose a folder for the new file. You may choose any folder or sub folder under the Local files folder. Press the blue Choose folder at the bottom of the window once you have navigated to the appropriate folder.
You can also change the format of the current file by choosing the appropriate format from the Export Format field. To create the new file with the filename and location that you have chosen, press the Save button.