• Go to Transactions > Purchase > Modify Bill.

• A 'Modify' window will appear.
• The user will select the options as per the Requirement i.e. bill no, bill date etc.
  

• A 'Purchase As On' window will appear.
• The user will press 'Enter' key on selected entry.

• A 'Purchase Alteration' window will appear.
• Click on 'Modify'.

• Now, the user will press 'Tab' key at the end of the Item.

• The user will keep pressing 'Enter' key.
• Then select 'Tax Summary'.

• A 'GST Summary' window will appear.
• The user will change the selected HSN Tax amount as per the requirement. 
• Suppose select HSN '12345678' with '6+6' Tax Amount. Press 'Enter' key.

• Suppose change the Tax to '9+9'.

• Then Press 'Tab' key to save changes.

• Now, click on 'Save'.

• Then keep pressing 'Enter' key.
• The user will select 'Yes' to save changes.

After following the above steps, the user can easily change HSN wise Tax in Purchase Bill.