Hello alimert, 

I'm happy to help you. 

To change default GST, you may go to your company settings.  Here's how:

1. Click on the gear icon on the top right. 
2. Go to Company Settings or Account and Settings.
3. Click on Advanced tab.
4. Under Accounting, click on the pencil icon to edit the Default tax rate selection.
5. Click on Save > Done. 

That shall do it.  Thanks for dropping by the Community.