UPDATE Statement With Multiple Criteria

I am trying write a query to update a column of data in my xLegHdr table however the update is based on multiple criteria. I was trying to use "IF..ELSE" statements but that is not working.

I would like to update the "SMiles" column based on the data in the "Dist" column. If the number in the "Dist" column is less than 250 then subtract 25 and multiply it by 1.15 the result should go in the "SMiles" column. If the number is grater than 250 then subtract 40 and multiply by 1.15 and place the result in the "SMiles" column; like so:

UPDATE xLegHdr
SET SMiles =
IF Dist<250 THEN Round(Dist-25)*1.15)
ELSE Round(Dist-40)*1.15)
END IF

Any ideas?