Currently the fastest way to rename a tag is to add a new tag and delete the old one to the same set of members.
Step 1: Add a Tag Filter
Head to Pool >> Home >> Add Filter, and then add the tag under the has specific tag(s) to view all members with that tag.
Step 2: Select all
Click the checkbox at top left to select all the results of your filter.
Step 3: Click the tag button
Now in the tag modal, type the renamed tag, and then remove the older tag name by clicking the blue X.
Success! Your filter may have zero results
Depending on your old tag, you may now see zero results here. That means it worked. Simply edit the filter to contain your new merged tag to verify everything worked correctly.