Is there a command to make first letter upper case?

Yes, using the mfirstuc package.

\documentclass{article}
\usepackage{mfirstuc}
\begin{document}
\capitalisewords{first letter upper case}
\end{document}

Moreover, one can exclude some words (as "a", "an", "the", "at", "by", "for", etc.) to follow capitalization styles with \MFUnocap{word}

\documentclass{article}
\usepackage{mfirstuc}
\MFUnocap{are}
\MFUnocap{or}
\MFUnocap{etc}
\begin{document}
\capitalisewords{first letters are upper case or lower case, etc}.
\end{document}

In this case, the result should be:

First Letters are Upper Case or Lower Case, etc.

However, note that this package use plain spaces to determine what is a word, so if you exclude "case" will work only in the first match, because the second time the word is really "case," (with a comma).

Alternatively, the titlecaps package can perform the same task but with taking care of the punctuation signs:

\documentclass{article}
\usepackage{titlecaps}
\Addlcwords{are or etc}
\begin{document}
\titlecap{first letters are upper case or lower case, etc.}
\end{document}

The output is exactly the same, but note that the final period was inside the capitalized string and "etc" was correctly not capitalized.

Also note that your mileage may vary with both packages if there are text formatting commands inside the text string, that make the capitalization task much more complex. For example, both packages manage well \textit{xxx}, \emph{xxx} fail only with titlecaps, \itshape xxx fail with only mfirstuc but {\itshape xxx} fail in both packages (although not in the same way).

you can use \MakeUppercase command,
when you write note={\MakeUppercase accessed:2.3.2019},
the output: Accessed 2.3.2019,
else if you write note={\MakeUppercase {accessed}:2.3.2019},
the output: ACCESSED 2.3.2019,

for example:

@misc{ref5, 
title={Use of Images}, url={https://www.hpl.ca/articles/use-images},
note = {\MakeUppercase  Accessed: 25.10.2019}}