You can make text superscript or subscript by adjusting its baseline.
Select some text, choose Format > Font > Baseline, and then choose an option from the submenu.
Superscript: Moves the text above the level of normal text.
Subscript: Moves the text below the level of normal text.
Raise: Moves the text up only a little bit.
Lower: Moves the text down only a little bit.
Use Default: Places text at the same level as the body text.
You can choose Raise or Lower repeatedly to nudge the text up or down to the position you want.