You need to follow below steps to Print Invoice on letter head:
1. Go to Settings > Template Settings >
Header > Company Header > Display Company Header: Disable
2. While you are printing from Book Keeper Windows you can change the margins.
Click View Invoice, Click on Print button.
On print preview window click top 4th icon on top (Page Setup) icon and then increase top margin to same of your letter head print length.
If you are using Book Keeper Android then you need to go to:
Settings > Template Settings > and change Extra Lines At Top Of Invoice.
Now you can take the print on your letter head.