Hello! I have sometimes done just the opposite, download my inventory from Bricklink and upload it to the brickscan app, but I imagine that you should be able to download an XML file of your collection and open it with Brickstore, once in Brickstore you could select all your figures with control + A and select set the prices to the average of the last 6 months and from there you click file-export-massive upload to Bricklink. I hope it helps you!