In this question, it is explained how to change table of contents numbering to Roman numerals in beamer: change numbering in table of content for Beamer.

I would like to change it to uppercase letters. How could this be done?

Change the initial part to

\documentclass{beamer}
\usepackage[utf8]{inputenc}

\defbeamertemplate{subsection in toc}{bullets}{%
\parbox[t]{1em}{\textbullet\hfill}%
\parbox[t]{\dimexpr\textwidth-1em\relax}{\inserttocsubsection}\par}

\makeatletter
\defbeamertemplate{section in toc}{sections numbered roman}{%
\@Alph\inserttocsectionnumber.\ %
\inserttocsection\par}
\makeatother

\setbeamertemplate{section in toc}[sections numbered roman]
\setbeamertemplate{subsection in toc}[bullets]

\begin{document}