I never remember how to do this. My e-mails are categorised in "Social" or "Promotions". I would like these e-mails to be organized into "Primary".
What is the easiest what to do this?
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Assuming you have a suitably modern browser simply drag the message from the message list and drop it on the tab where you'd like it to be organized for the future. (You should be prompted by Gmail whether or not you want future messages from the same sender to be added to the new category tab.)
If your browser doesn't allow for that, or if you want to move multiple conversations at a time, Inbox Categories are also listed with your custom labels in the "Move to" and "Labels" menus. Just check the messages you want to move and use the appropriate option.
More information from Gmail Support.