You have to go in the Articles module, once you have sorted your articles, you can select them either one by one or by selecting all the articles :

Once one or more items have been selected, it is then possible to see the button below appears on the left :

Once you have clicked on the Change price button, this window appears :

Several options are available :

• Price to change : You can choose between the Promotional Price and the Public Price.
• Operation : Modify price or remove price 
• Sales policy : Select the desired calculation
• Coefficient : Choose the desired coefficient

then click on Finish :

To import a list of items, please :

• Click on the top menu Items module
• Click on the left menu All 
• Click on the three-dot menu on the right to access Import an item list

   

A window appears for you to go get your file in CSV format.

You have three choices : 

Depending on your needs, click on one of the possible options.

Then click on Next. 

It's perfectly possible to modify your prices in this way, by filling in your file with new prices.