If I update a vendor’s TDS , will the new TDS appear on my previous Purchases?

NO!! If you update TDS of any Vendors ,the updated TDS of the Vendor will not reflect in your previous Purchases.

To do so please follow the below mentioned steps:

Step 1: Click on Edit under Vendors.

Step 2 : Enable the toggle button for TDS and select TDS percentage.
Now, click on Update Vendor.

Step 3: This is how outcome will look like: