RemNote offers two options for adding superscripts and subscripts to your notes.
Use the direct superscript/subscript format options. You can find these on the … menu after selecting some text, or you can press Ctrl+Shift+. and Ctrl+Shift+, respectively (Cmd+Opt+. and Cmd+Opt+, on a Mac).
This method is convenient if you have some very simple math or chemical formulas, or are using the superscripts or subscripts for footnotes or other non-mathematical purposes.
Use LaTeX equations. For more complicated math, you'll get much nicer formatting for both your superscripts/subscripts and the rest of your equation by using LaTeX. Type
$$to start entering a LaTeX equation.
See Writing Equations with LaTeX for a detailed treatment of this feature.