Stocks page at inventory section shall show you list of all Variants with stocks quantity. You can inward purchases and manage stocks of the variants clicking 'Manage Stocks' button.
Add 'Reference date' (supplier invoice date) and 'Reference number' (supplier invoice number). Select 'Add' for stock inward or any other as per update type. For add and return the supplier name is mandatory.
Search the variant need to be updated.
You can add new stocks to existing batch if the pricing is same, else create a new batch with new buying price, selling price, tax and discounts.
Upon saving, the update shall registered and recorded in the variant history.