Though Oracle hasn’t provided straight forward method to disable TDE . But there is a work around for this.

NOTE – Don’t implement this on production database. Oracle Support/Development team will not help in resolving any issues arising due to such operations. 

Follow Below steps
 
Find the encrypted table columns and modify them:
 

SQL> select owner,table_name,column_name,encryption_alg from dba_encrypted_columns;

OWNER TABLE_NAME   COLUMN_NAME	 ENCRYPTION_ALG
----- ------------ ------------ -----------------------------
RAJ   EMPLOYEE	   SALARY	 AES 192 bits key


SQL> alter table raj.employee modify( salary decrypt);

Table altered.

SQL> select owner,table_name,column_name,encryption_alg from dba_encrypted_columns;

No rows selected

Now find the table under encrypted tablespace and move to normal tablespace:

SQL> select segment_name,tablespace_name from dba_segments where tablespace_name in ( select tablespace_name from dba_tablespaces where encrypted='YES');

SEGMENT_NAME				   TABLESPACE_NAME
---------------------------------------- ------------------------------
ENC_TABLE				   TEST_ENCRY



SQL> alter table enc_table move tablespace users;

Table altered.

SQL> select segment_name,tablespace_name from dba_segments where tablespace_name in ( select tablespace_name from dba_tablespaces where encrypted='YES');

no rows selected



SQL> drop tablespace test_encry including contents and datafiles;

Tablespace dropped.

switch logfiles multiple times

SQL> alter system switch logfile;
System altered.

SQL> /
/
/


System altered.

SQL> 
System altered.

Close the wallet, move the wallet files and restart the database:

SQL>ADMINISTER KEY MANAGEMENT SET KEYSTORE close;


cd /media/sf_stuff/wallet
mv * backup/

SQL> startup force

Now TDE has been disabled. Now if you want to enable TDE again, then restore the wallet files and open the key.