After the death of Queen Elizabeth II many things will start to change throughout the United Kingdom.
The national anthem will of course change. "God save the Queen" will change to "God save the King".
The stamps currently feature the queen's portrait and will soon feature the king as they are brought into circulation. Postboxes in the UK also have the cypher, "EIIR" written on them, which means Elizabeth II Regina (regina meaning king or queen in latin). However they will likely not be replaced, but as new ones are created they will have the king's cypher on them instead.
The money, known as pound sterling, also currently features the queen's portrait but they will eventually be replaced with the king's. The queen's portrait also appears on some currency in other countries like Canada and Australia, but nobody knows yet if those countries will keep them as they are or replace them with the king's portrait.
Many of the Royal family's titles are likely to change too. Prince William, known formerly as the Duke of Cambridge, will now be known as the Duke of Cornwall and Cambridge. Many more titles will likely change.
The biggest change of course is that the UK now has a new king. Of course, whether he will be as liked and as fondly remembered as the queen remains to be seen.
AH