I'm here to help you upgrade your QuickBooks Desktop from 2012-2021, @paul83.

As an insight, upgrading your company file from the 2012 version to QuickBooks Desktop (QBDT) Pro 2021 is a big leap. The QBDT program has a 3-year life cycle. 

That means you have to convert your data file(s) in stages from 2012 to 2015 to 2018 to 2021. Then, restore the file in between. You need to find and install the trial license of QBD 2012 and 2015 before converting the file(s) to QBD 2021. 

You can refer to this article's Step 3 section for the step-by-step guide: Upgrade your company files after switching to a new version of QuickBooks Desktop.

In the meantime, I'm adding this article to help you get familiar with the program: QuickBooks Desktop user guides. It contains do-it-yourself articles, training, and pro advice links to name a few.

I'm just around to help if you need further assistance in converting your files, @paul83. Take care!