I made some amendments to last year's BB invoices this year. How do I show these amendments properly in BUSY?

To show amendments made to last year's B2B invoices in BUSY, you need to update the invoices in BUSY and save them. The changes will then be reflected in the updated invoices.You can run Update Voucher GST Summary but you need to follow some steps:
Step 1- Take the backup of your concern company.
Step 2- Open your company and go to house keeping 
Step 3- After click on house keeping Click on the Update Voucher GST Summary.
Step 4- Click on the partial or can select full and run the process.
If you have made change in bill sundry,Sale type etc then you need to run Regenerate Dual Voucher for purchase voucher and you can follow these steps:
Step 1: take backup of your concern company.
Step 2: Now open your company and click on House Keeping.
Step 3: Now click on Regenerate Dual Voucher then it will show you the prompt for Take backup there you can Tick for option you can proceed as I have the latest backup.
Step 4: Now apply filter in which you have made changes otherwise tick on None option.
Step 5: Now Filter on date and selected voucher as per your requirement and click on OK Option. After completion of process you can check your data.