First - Does this computer Qualify for Mojave 10.14 - https://support.apple.com/kb/SP777?locale=en_CA
If YES - follow this link. Be certain to use Safari in Links in following page as Others may not function. Safari will open a Redirect to open Apple Store where you can download Mojave.
Very Important - before attempting - make a Time Machine Backup.
Also Important - if you have any AntiVirus Software installed now - Remove it ONLY via the Developers Instructions. This Software is not needed, interferes with the normal operation of the OS, causes havoc and Will Interfere with the upGrade Process.