If you are using Photoshop:
To edit text in a type layer:
1. Select the Horizontal Type tool "T" tool or the Vertical Type tool "T" .
2. Select the type layer in the Layers palette, or click in the text flow to automatically select a type layer.
3. Position the insertion point in the text, and do one of the following:
* Click to set the insertion point.
* Select one or more characters you want to edit.
4. Enter text as desired.
5. Commit the changes to the type layer.
This is from the Photoshop help hehe ^_^