Oracle SQL - replace null with previous known value

Question

I am looking to replace null values from my table's column with the previous known value from that column. I have tried and searched many things but I can't seem to figure out how to do it. If someone could help that would be great. Thanks in advance!

PROCESSO    DATAM   HOURM TEMP_T1
-------- ---------- ----- -------
508410   2015.03.29  19   36.15
508410   2015.03.30  11   NULL
508410   2015.03.30  14   NULL
508410   2015.03.30  15   35.36
508410   2015.03.30  23   36.52
508410   2015.03.31  11   NULL
508410   2015.04.01  02   37.06

I want it to appear like this:

PROCESSO    DATAM   HOURM TEMP_T1
-------- ---------- ----- -------
508410   2015.03.29  19   36.15
508410   2015.03.30  11   36.15
508410   2015.03.30  14   36.15
508410   2015.03.30  15   35.36
508410   2015.03.30  23   36.52
508410   2015.03.31  11   36.52
508410   2015.04.01  02   37.06

Answer 1

Try last_value analytic function:

SELECT "PROCESSO", "DATAM", "HOURM",
       last_value( "TEMP_T1" ignore nulls )
       OVER (order by "DATAM"
             rows between unbounded preceding and current row
       ) as new_temp
FROM table1

Demo ==> http://sqlfiddle.com/#!4/48207/2

=========== EDIT ===================

If you want to update the table, and there is no primary key (unique identifiers), you can try a solution based on rowid pseudocolumn
==>http://docs.oracle.com/cd/B19306_01/server.102/b14200/pseudocolumns008.htm) :

MERGE INTO table1 t1
USING (
    SELECT rowid rd, "PROCESSO", "DATAM", "HOURM",
           last_value( "TEMP_T1" ignore nulls )
           OVER (order by "DATAM"
                 rows between unbounded preceding and current row
           ) as new_temp
    FROM table1
) x
ON (t1.rowid = x.rd)
WHEN MATCHED THEN UPDATE SET t1."TEMP_T1" = x.new_temp
;

Demo ==> http://sqlfiddle.com/#!4/5a9a61/1

However you must ensure that there is no another process that deletes and inserts rows from/into this table while running the update, because when a row is deleted from the table, Oracle can assigng it's rowid to another, new row.