Select the cell or text you want to format
Press Ctrl+1 to open Format Cells
Go to the Font tab
Check Subscript
Click OK
For partial text in a cell, double-click the cell or press F2
Highlight the specific characters you want as subscript
Press Ctrl+1
Check Subscript
Click OK
On Mac, use Command+1 instead of Ctrl+1
If needed, use the Equation editor for more advanced formatting