Do the following.
1. Close the sdk manager and eclipse.
2. Go to the folder where you have stored your adt.
3. In that adt folder you'll find a folder known as tools.
4. Make a copy of the contents of that folder and paste it in a folder called copytools.
5. Now go to the command prompt and go to the location of the copytools.
6. Then execute the command android.bat the sdk manager will start.
7. Now update all the plugins you want. It'll update your original folder.
8. After the update delete the copy.