How to update a SharePoint list item using REST

If the list’s forms are customized and you can’t edit a field using them, with due permissions you can just make a fetch request from the console to update a certain field.
You need the list name, item ID, field internal name and the new value.
As with all POST requests to SharePoint, you need the REQUESTDIGEST value, which you can get from a freshly loaded page, by searching the page contents by `__REQUESTDIGEST`
The request you need to make is:

fetch("/sites/SITE_NAME/_api/web/lists/getbytitle('LIST TITLE')/items(ITEM_ID)", {body: JSON.stringify({FIELD_INTERNAL_NAME: 'NEW_VALUE'}), method:'POST', headers:{'content-type': 'application/json',accept:'application/json;odata=nometadata', 'X-HTTP-Method': 'MERGE', 'X-RequestDigest': 'REQUESTDIGEST', 'If-Match': '*'}}).then(a => a.text()).then( a => console.log(a))