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

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

 

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

 

 

switch logfiles multiple times

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

 

 

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