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}]
– Nicola Talbot Jan 17 '14 at 12:06\renewcommand*\acronymname{Abbreviations}
? – cgnieder Jan 17 '14 at 12:10babel
may reset it if the language switches. There's more detail in Changing the Fixed Names – Nicola Talbot Jan 17 '14 at 12:40babel
may change it. The question isn't very specific but unless I'm mistaken it reads like the OP doesn't (want to) usebabel
... – cgnieder Jan 17 '14 at 12:44