You can edit the tag tree by going to the main page of your site, and then clicking on the main 'ask a question' button.
As moderator, you should see 'edit categories' above the tag tree.
You can edit, add or delete tags. As tags are hierarchical, if you delete the top level of the tree, it will delete the tags underneath (e.g. deleting 'hygiene' will delete all sub-tags). Please note, the tag tree is a representation to help structure tags and help enquirers to use existing tags; deleting a tag here doesn't actually delete the tags already used in questions.
The image below is what happens when you hover your mouse over a tag (in this screenshot, it's 'human_rights'). When I do this, the words 'edit' and and 'x' (for delete) appear:
If you want to add a tag under CLTS, first click on 'sanitation', then its subcategory 'CLTS', and then I click on 'add tag'. You should also see a faint yellow outline on the categories you have clicked, which show you where you are in the hierarchy.
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