For adding a signature to an existing invoice, follow the below steps.

• Select an invoice to open it
• Click on the 'S' sign to add a signature 
• A popup will appear on the screen 
• Add name, title, date, and signature.
• To add a signature, you can use the mouse pointer by adjusting the thickness. 
• Click on done to add signature

After adding a signature, you can check it on your invoice.