Using PL/SQL, in a loop through all records, apply TO_DATE function to that column (using format mask you find appropriate). Put exceptions into another table. Something like this:
for cur_r in (select id, date_value from your_table) loop
l_date := to_date(cur_r.date_value, 'dd/mm/yyyy');
when others then
insert into log_table ...
Note that use of WHEN OTHERS is usually not recommended, but in this case (as you don't really care which error it is - value simply isn't in a format you expected it to) it is acceptable. If you wish, you can develop this suggestion and include various format masks, exception handlers etc.
Anyway, if possible, modify that column to DATE data type.