Enable categories if you haven't yet, see Help&Support.
Select "All Transfers" as the category. Then click once in the torrents list. Press CTRL+A to select all. Right-click -> Trackers -> "Edit" OR "Find and replace" OR select a preset. You can edit the presets in Settings. For example you would create a "Free public trackers" preset and add the impartial trackers there.
Usually you do not need to do this because DHT works just fine. If the torrent finds peers then you don't need anything.
DO NOT add public trackers to any private torrents, unless you are the last peer on earth who used that particular private tracker.
