You should always consult Oracle documentation first. What you ask for is a straightforward, single-SQL-command-involving action. The crucial piece of knowledge you missed is that you do not alter a tablespace, you alter a datafile.
Proof of concept
First, I'll just query my example tablespace for its block size, as I'll use the value for proving that my datafile was altered correctly.
SQL> select tablespace_name, block_size
SQL> from dba_tablespaces
SQL> where tablespace_name = 'EXAMPLE';
TABLESPACE_NAME BLOCK_SIZE
------------------------------ ----------
EXAMPLE 8192
OK, the tbs uses a block size of 8KB.
Now, what my example data files look like?
SQL> select file_name, file_id, tablespace_name, autoextensible, increment_by * &example_tbs_block_size_b / 1048576 as increment_by_mbytes
SQL> from dba_data_files
SQL> where tablespace_name = 'EXAMPLE';
FILE_NAME FILE_ID TABLESPACE_NAME AUTOEXTENSIBLE INCREMENT_BY_MBYTES
---------------------------------- ---------- --------------- -------------- -------------------
D:\ORA\MY_CDB\MY_PDB\EXAMPLE01.DBF 10 EXAMPLE YES 1
OK, I see just a single data file with the autoextend of 1MB.
Now alter the datafile...
SQL> alter database datafile 10 autoextend on next &target_autoextend maxsize unlimited;
Database altered
And recheck the tbs datafiles once again
SQL> select file_name, file_id, tablespace_name, autoextensible, increment_by * &example_tbs_block_size_b / 1048576 as increment_by_mbytes
SQL> from dba_data_files
SQL> where tablespace_name = 'EXAMPLE';
FILE_NAME FILE_ID TABLESPACE_NAME AUTOEXTENSIBLE INCREMENT_BY_MBYTES
---------------------------------- ---------- --------------- -------------- -------------------
D:\ORA\MY_CDB\MY_PDB\EXAMPLE01.DBF 10 EXAMPLE YES 8
And, voilá, I have an autoextend of 8MB.