This answer may not be useful or true if my assumption is wrong - when you say "tag", I have assumed you mean what Jira calls "labels"
To delete a tag, you have to remove it from every issue it has been added to.
There is no "edit" function for tags, you have to add a new one and delete the old (again on all issues it has been applied to)
You might want to have a look in the Marketplace though, there are some apps which add some tagging functions, and I know Scriptrunner can do search and replace (not quite "edit", but has the same outcome)
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