Hi,
I think I just answered your question on the other page, but Ill post it here as well incase anyone else has the same question!
Its really up to you, you can download and open the new sheet and transfer the previous years sales (follow the instructions on the account information tab) and keep everything in one place...
OR
You can open a new sheet and only start downloading new sales starting with 2018 (keep them separate).
Personally I keep my sales combined because the reports will automatically separate the data by year, but if you are a high volume seller it may be more efficient (quicker program response times) to keep separate sheets for each year (your preference).
Let me know if you need further assistance on this topic.