I have an 24 in iMac from 2007 with OS X Yosemite 10.10.5 and I need to update the software to at lease OS X 10.14. Is this possible and where can I get the software update download?

You can only upgrade as far as El Capitan (10;11.6) with that era iMac. You need a Late-2012 or later model to get to Mojave. See How to get old versions of macOS using your Safari browser.

In short, it is time for a new iMac:

• A late 2009 to 2011 iMac can run up to High Sierra V10.13.6
• A 2012-2013 iMac can run up to Catalina V10.15.7
• 2014 or later iMacs can run current versions of Mac OS, the current version is Big Sur 11.2

Your 2007 simply does not have the hardware to support anything later the 10.11.6.

If you cannot afford a new iMac then there are many alternatives, if you are considering a new(er) Mac please let us know and we can give you some ideas on new, refurbished and good quality used Macs.

Not a clue really, you may want to check with Microsoft to see what if anything they can do for you.

If it is OutLook from Microsoft 365, then Microsoft will require that you are running macOS Mojave (10.14.6) or later to install it.