Question : Want to update a field with values from another field within the same table if it meets a condition

I have: Tablegz

Novice I am at SQL.  Using Access 2007.

So I want to fill field FINGP with values from field PROP if PROP has values, everything is within the same table.  Note that field PROP does have cells with no data--are empty in other words, not even a zero.  If PROP does not have values then FINGP should get filled with the values from field EXG.  Right now I get an error for missing an item using the first statement below.  Do I even have the right expression for what I am trying to accomplish?  Is an IIF statement allowied in an UPDATE?

UPDATE Tablegz
SET ([FINGP]=([EXG], iif([PROP]='')), [FINGP]=[PROP]

or is this more appropriate?   SELECT Tablegz, IIF([PROP]=' ', [FINGP]=[EXG]), [FINGP]=[PROP]

However the second expression here asks me to enter a parameter value and if I enter nothing and press OK 3 times it returns 3 empty fields in the following order: Tablegz, Expr1001, Expr1002.

I don't know folks.  I need more practice big time.

Your expertise is appreciated.

Answer : Want to update a field with values from another field within the same table if it meets a condition

UPDATE Tablegz
SET FINGP = IIf(Nz(PROP, "") <> "", PROP, EXG)