How do I upgrade a 2013 Powerbook Pro from El Capitan to Mojave?

I want to upgrade my laptop (2013) to Mojave from El Capitan, as support for El Capitan is dropping away – but it would be counterproductive for me to go further then Mojave as I will lose access to a lot of 32-bit software. However, instructions (seemingly oh-so-simple) fall down because they send me to the App Store, in which I can find none of the Mojave download software touted by 'helpful' lists of instructions. Where am I going wrong, or can't it be done?