Omit the where clause:
update mytable set
column1 = value1,
column2 = value2,
-- other column values etc
;
This will give all rows the same values.
This might not be what you want - consider truncate then a mass insert:
truncate mytable; -- delete all rows efficiently
insert into mytable (column1, column2, ...) values
(row1value1, row1value2, ...), -- row 1
(row2value1, row2value2, ...), -- row 2
-- etc
;