You can edit existing text by accessing its Properties (see Object Properties). Open the General page, and edit the text in the Attribute field.
You can also edit the content of the text by clicking on the text with the Text tool, or, if in the Selector General properties the Double Click Action is set to Edit content, you can edit the text content by double clicking on the text with the Selector tool.
If you use the Selection Info Palette, you can use the Edit in Place tool to edit text directly on the screen. See Selection Info Palette. In this palette, the text is also listed (and can be edited) in the General category, next to Info.
You can use Explode to explode text into individual characters, and explode characters into polygons and polylines, and explode these into lines. This can be useful if you need to change a very small aspect of text, or if you need a text-shaped polyline to extrude.
See Exploding Objects.