If you want to replace one tag with another, across all assets that contain that tag, here's how:

First, search for the tag you wish to replace, either using the search box or by clicking on the tag on any image that has that tag (in this example we are going to replace the tag "fog" with "mist");

Then, select all the assets and click edit, and then remove the "fog" tag and add the "mist" tag, then press Done.
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Tip! You can also use this technique to add additional tags in bulk, for example if you wanted to add "mist" to all assets with the tag "fog", you could do so using this method (just skip the part where you delete the tag!)