How to update existing issues via REST?

Sorry if this question sounds stupid, but the REST API Reference provides a lot of information about how to create or delete items like issues, tags, attachments, but I cannot find any examples explaining how to update an existing issue. Maybe it is too simple or obvious for me to see, but I have no clue how to do that.
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I have a question about adding relation tasks. How I can add link to some task at some task via rest api? This page:

Doesn't answer me for this question.
For example. I have task EX-123, and I have subtasks EX-122 and EX-121. How I can add link to EX-122 and EX-121 at EX-123 as "relates to".

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