First thing is that the word Tag has two usages in Calibre - a Tag as in a Keyword, and a more generic usage, where Tag means all the ways of 'labelling' a book - Author, Publisher, Series... and of course Tags (as in keywords) - it took me ages to figure that out.
First thing is to make sure the Tag Browser is showing - it should be on the left, if not click on the icon that looks like a luggage tag in bottom right corner.
To Change "Fictoin" to "Fiction" - locate "Fictoin" in the Tag Browser, right click it. At the top you'll see "Rename Fictoin", click it, change it to "Fiction" (its an in-line edit) and you're done.
There are 2 other ways I can think of to do that, and there probably others - but that's the way I'd do it.
Good luck BR