Usually you change this with \documentclass, yet it only werks with the range
[8 - 12]. So just use KOMA.
\usepackage{scrextend}
\KOMAoption{fontsize}{14pt}
You can also dynamically change the fontsize with \fontsize.
\fontsize{36}{39}
Ref: how-to-specify-font-size-less-than-10pt-or-more-than-12pt on tex.stackexchange.com
You can also define a font to use later in your code. I found that this works
out with installed .ttf files as well, that’s why I can write with the
TempleOS font.
\usepackage{fontspec}
\newfontfamily\mynewfont{TempleOS}