If you want to modify one or more characters in a text layer differently than the rest, you could turn the text into a shape. Then modify the points in the path to give them a new shape.
To convert text into a shape, right-click on the text layer, and choose “Convert To Shape”. Then select the Direct Selection tool (the white arrow tool) by pressing Shift A and click-and-drag the points in the path to give the characters a new shape. Remember that once you convert your text, it will no longer be editable.