you cannot move contents from one datafile to another, you can do it at tablespace level. If you want to move the contents of that datafile to another place you have some options

in Oracle 8i
You can do alter table XXX move tablespace ZZZ, this moves one table from one tablespace to tablespace ZZZ. Do this for all your tables and drop the old tablespace
For indexes you do alter index XXX rebuild ZZZ

in Oracle 8 you probaly have to do Export/Import for tables as for indexes you can still use above option

If what you want to to move one datafile from one directory to another you shutdown the database copy the file from one place to another then startup mount and do a rename of datafiles, I think ths syntax is

ALTER DATABASE
RENAME FILE '/u02/oracle/rbdb1/sort01.dbf',
'/u02/oracle/rbdb1/user3.dbf'
TO '/u02/oracle/rbdb1/temp01.dbf',
'/u02/oracle/rbdb1/users03.dbf;