To rename a tag:

  1. Bring up the Tags screen for a page (it doesn't need to be a page tagged with the tag you want to rename)
  2. Tap the Edit button (top right hand corner of the screen)
  3. Tap the tag you want to rename
  4. Edit the tag name
  5. Tap the Save button (top right)
  6. The Tags screen should reappear,  then tap Done
  7. Tap Save to exit the Tags screen

NOTE: In Trunk Notes 4.6.0 and below there is an issue with Dropbox synchronisation when renaming tags. Pages do not get updated on Dropbox with the new tags.