How to replace text

\def\replace#1#2#3{%
 \def\tmp##1#2{##1#3\tmp}%
   \tmp#1\stopreplace#2\stopreplace}
\def\stopreplace#1\stopreplace{}

\replace{Text should be replaced here, here and here}{here}{Latex}

\bye

written as plain tex but would work in latex too.

This uses the higher-level macro StrSubstitute from xstring package. Use [0] as first optional parameter to replace all occurences of here, but as egreg stated in a comment, it will also replace in words like where or there

\documentclass{article}

\usepackage{xstring}




\begin{document}

\StrSubstitute[0]{Text should be replaced here, here and here}{here}{Latex}
\end{document}

A complex solution that only replaces complete words:

\documentclass{article}
\usepackage{xparse,l3regex}

\ExplSyntaxOn
\NewDocumentCommand{\replace}{mmm}
 {
  \marian_replace:nnn {#1} {#2} {#3}
 }

\tl_new:N \l_marian_input_text_tl
\tl_new:N \l_marian_search_tl
\tl_new:N \l_marian_replace_tl

\cs_new_protected:Npn \marian_replace:nnn #1 #2 #3
 {
  \tl_set:Nn \l_marian_input_text_tl { #1 }
  \tl_set:Nn \l_marian_search_tl { #2 }
  \tl_set:Nn \l_marian_replace_tl { #3 }
  \regex_replace_all:nnN { \b\u{l_marian_search_tl}\b } { \u{l_marian_replace_tl} } \l_marian_input_text_tl
  \tl_use:N \l_marian_input_text_tl
 }
\ExplSyntaxOff

\begin{document}

\replace{Text should be replaced here, here and here}{here}{\LaTeX{}}

\replace{Text should be replaced here, here and not there}{here}{\LaTeX{}}

\end{document}

A simpler solution that replaces all occurrences:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\replace}{mmm}
 {
  \marian_replace:nnn {#1} {#2} {#3}
 }

\tl_new:N \l_marian_input_text_tl

\cs_new_protected:Npn \marian_replace:nnn #1 #2 #3
 {
  \tl_set:Nn \l_marian_input_text_tl { #1 }
  \tl_replace_all:Nnn \l_marian_input_text_tl { #2 } { #3 }
  \tl_use:N \l_marian_input_text_tl
 }
\ExplSyntaxOff

\begin{document}

\replace{Text should be replaced here, here and here}{here}{\LaTeX{}}

\replace{Text should be replaced here, here and not there}{here}{\LaTeX{}}

\end{document}