If a Tag has been entered by mistake, for example, the spelling is wrong, how can the Tag be changed?
Additionally to removing it from all questions as Chenmunka suggested, a moderator can fix this directly:
The tag can be merged into one of the correct spelling. Let's say somebody created prove-of-stake, and obviously it should be proof-of-stake.
When the moderator merges prove-of-stake into proof-of-stake all questions with the former get retagged to proof-of-stake.
The usual way to ask for moderator help is to a) open a ticket on meta and tag it with support, or b) to flag a question with "Needs moderator attention" and give a sufficient explanation there.
If it is only on a very small number of questions (<5), it's probably easiest to just retag all of those questions, when it's on more questions it is probably better to request moderator support, as all edited questions get bumped to the frontpage needlessly otherwise.
A similar effect can also be reached by suggesting synonyms: Let's say both pos and proof-of-stake exist. Only the latter should be kept, but people will surely use the former again. You would go on the Tag Info of pos and click "suggest synonym". When other users vote to accept the synonym, pos gets replaced to proof-of-stake on all questions and whenever it is applied later.
To answer the 'remove' part of your question.
The system automatically deletes tags that have no questions. This is done by a task that checks every now and again. See this post on SE.Meta.
Just remove the tag from all questions and wait.
Click the 'edit' link under the question. You'll get the post input UI back, along with the tag input section at the bottom.