You mean you want to replace
<a b="c"/>
with
<a b;"c"/>
? You can't do that: the output has to be well-formed XML.
Oh, I guess by "inside a tag" you probably mean "between a start tag and an end tag" - for example <a>xxxxx = yyyyy</a> becomes <a>xxxxx ; yyyyy</a>. It really does make things simpler if you use terms correctly. That "=" sign is not inside a tag.
The answer is simple: use the translate function
<xsl:value-of select="translate(., '=', ';')"/>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference