PredString = '99'

conn = pymssql.connect(server="MyServer", database="MyDB", port="1433", user="****", password="******")
dfUpdate = pd.read_sql("EXEC UpdatePredictions '" + PredString + "'", conn)
conn.close()

print(dfUpdate)

alter procedure UpdatePredictions (@PredString varchar(max)) as
begin

    update MyTable set PredMths = @PredString

    select 'Updated.'

end

EXEC UpdatePredictions '99'