Replacing some characters in a string with another character
You might find this link helpful:
http://tldp.org/LDP/abs/html/string-manipulation.html
In general,
To replace the first match of $substring with $replacement:
${string/substring/replacement}
To replace all matches of $substring with $replacement:
${string//substring/replacement}
EDIT: Note that this applies to a variable named $string.