Hi,
you could try

select cast (extract(day from datefield) || '.' || extract(month from
datefield) || '.' || (extract(year from datefield)-100) as date) from
mytable

or update mytable set datefield = cast (extract(day from datefield) ||
'.' || extract(month from datefield) || '.' || (extract(year from
datefield)-100) as date)

I'm not sure what would happen to leap years though.

Regards
Ian Newby