There are several ways to accomplish this.
The simplest is to type 201/2, highlight the "1/2", and under Format > Font
select Superscript. This does not give you the standard 1/2 symbol, but may
be OK.
If you have AutoFormat set to convert 1/2 to the symbol version (see Tools>
AutoCorrect, the AutoFormat tab), type is 20 then space then 1/2 then space.
The 1/2 will convert to the symbol. The delete the space between 20 and the
1/2 symbol, select the 1/2 symbol and use Format > Font and Superscript
again. This is probably the optimum version.
A third approach is to insert a Field. Insert > Field > Equations and
Formulas:
Select Eq, then Options, go down to \S() and click Add to Field. In the
parentheses type 1/2 so that the field reads EQ \S(1/2), then click OK. You
can then highlight the resulting 1/2 and change the font as desired. Again
this is not the real 1/2 symbol, but might be OK.