I am not sure but I guess that you want to decrease the bottom margin?
With an up-to-date KOMA-Script version you could set usegeometry as class option. Then you can load geometry and set either footskip=0pt or option nofoot.
\documentclass[a4paper,11pt,
usegeometry% <- added
]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage[footskip=0pt]{geometry}% <- added
\usepackage{fancyhdr}
\fancyhf{}
\pagestyle{fancy}
\fancyhead[R]{Page \thepage}
\fancyhead[L]{\nouppercase{\leftmark}}
%\usepackage{showframe}% to show the page layout
\usepackage{blindtext}% only for dummy text
\begin{document}
The footskip is \the\footskip.
\Blinddocument
\end{document}
But this does not change the size of the bottom margin. So I guess you want to do something like
\documentclass[a4paper,11pt,
usegeometry% <- added
]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage[nofoot,
bottom=2cm% <-
]{geometry}% <- added
\usepackage{fancyhdr}
\fancyhf{}
\pagestyle{fancy}
\fancyhead[R]{Page \thepage}
\fancyhead[L]{\nouppercase{\leftmark}}
%\usepackage{showframe}% to show the page layout
\usepackage{blindtext}% only for dummy text
\begin{document}
The footskip is \the\footskip.
\Blinddocument
\end{document}
Result:
