can I upgrade directly from os 10.13.6 to os 10.14.7?

That would depend on the age of your Mac. See Apple menu () > About This Mac, and on the description line, look inside the parenthesis. For Mojave 10.14.6, you will need a 2012 or newer iMac to do so.

If you pass that test, then you can get the full Mojave 10.14.6 installer from: How to download macOS - Apple Support which will place the installer in your /Applications folder.

Back up your iMac to a Time Machine drive before any upgrade, and it is a good practice to check with your printer/scanner vendor to ensure that they support these devices on Mojave.