You can do this by modifying the column and setting the identity properties:
create table t (
c1 int generated as identity
);
select cache_size, increment_by
from user_sequences
where sequence_name like 'ISEQ$$%';
CACHE_SIZE INCREMENT_BY
20 1
alter table t
modify c1
generated as identity
increment by 2
cache 1000;
select cache_size, increment_by
from user_sequences
where sequence_name like 'ISEQ$$%';
CACHE_SIZE INCREMENT_BY
1000 2