If you're not using babel
or translator
or polyglossia
then either my suggestion or @cgnieder's will work. (It would help if you could provide a minimal working example (MWE) when you ask a question.)
First option (see Displaying a Glossary in the user guide):
\documentclass{article}
\usepackage[acronym]{glossaries}
\makeglossaries
\newacronym{abc}{ABC}{a sample acronym}
\begin{document}
\gls{abc}.
\printglossary[type=\acronymtype,title={Abbreviations}]
\end{document}
Second option (see Changing the Fixed Names in the user guide):
\documentclass{article}
\usepackage[acronym]{glossaries}
\makeglossaries
\newacronym{abc}{ABC}{a sample acronym}
\renewcommand*{\acronymname}{Abbreviations}
\begin{document}
\gls{abc}.
\printglossaries
\end{document}
Both the above will produce:

If you do decide to use babel
(or if someone else happens on this question who wants a more general answer) either use option 1 above or:
\documentclass{article}
\usepackage[british]{babel}
\usepackage[acronym,translate=babel]{glossaries}
\makeglossaries
\newacronym{abc}{ABC}{a sample acronym}
\addto\captionsbritish
{%
\renewcommand*{\acronymname}{Abbreviations}
}
\begin{document}
\gls{abc}.
\printglossaries
\end{document}
In this specific case (use "Abbreviations" rather than "Acronyms") another possibility is to use the extension package glossaries-extra
:
\documentclass{article}
\usepackage[nomain,% don't create 'main' glossary
abbreviations% create 'abbreviations' glossary
]{glossaries-extra}
\makeglossaries
\setabbreviationstyle{long-short}
\setabbreviationstyle[acronym]{short}
\newabbreviation{html}{HTML}{hypertext markup language}
\newacronym{nato}{NATO}{North Atlantic Treaty Organization}
\begin{document}
\gls{html} and \gls{nato}.
\printglossaries
\end{document}

\printglossary
e.g.\printglossary[type=\acronymtype,title={Abbreviations}]
\renewcommand*\acronymname{Abbreviations}
?babel
may reset it if the language switches. There's more detail in Changing the Fixed Namesbabel
may change it. The question isn't very specific but unless I'm mistaken it reads like the OP doesn't (want to) usebabel
...