There is 2 ways: use binded editing or Bulk Editing.
- This way is good when you can do it for some products or variations on the same one page in the Products Editor
- Select some products
- Enable 'binded editing' switcher
- Hover on any selected product sell price cell and click on calculator icon
- In calculator select: rp- and in the text field there enter 20, and in the last drop-down select percents
- Select prices rounding if you need it
- Click on blue accept button on the calculator and wait until this bulk operation is finished
- If you did smth wrong you can roll back this operation in the tab History
- In this way you can apply changes to all products on the site or to the filtered ones by your logic
- So make products filtering by category or categories, or/and any other parameters if you need it
- Go to tab Bulk Edit and select field 'Sale price' there
- Enter 20 in the textinput
- In the drop-down select 'regular price minus value'
- Click on 'Do Bulk Edit' button and wait until operation is finished