TL;DR - The SQL UPDATE command allows you to modify the data records in the table. It is usually used together with the WHERE clause: if you don't use it, all the records in the table will be updated.
Contents
The syntax for SQL UPDATE
 Example    
UPDATE table_name
SET column1 = value1, column2 = value2, ...
WHERE condition;
Examples using a demo database
The Developers table
| ID | Name | City | Country | 
|---|---|---|---|
| 1 | Tom Kurkutis | New York | USA | 
| 2 | Ana Fernandez | London | UK | 
| 3 | Antonio Indigo | Paris | France | 
| 4 | Aarav Kaelin | Delhi | India | 
| 5 | Andrew Tumota | Miami | USA | 
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Updating a single entry
 Example    
UPDATE Developers
SET City = 'Berlin', Country= 'Germany'
WHERE Name = 'Antonio Indigo';
The result
| ID | Name | City | Country | 
|---|---|---|---|
| 1 | Tom Kurkutis | New York | USA | 
| 2 | Ana Fernandez | London | UK | 
| 3 | Antonio Indigo | Berlin | Germany | 
| 4 | Aarav Kaelin | Delhi | India | 
| 5 | Andrew Tumota | Miami | USA | 
Updating entries that comply with a defined condition
 Example    
UPDATE Developers
SET Name='Ben'
WHERE Country='India';
The result
| ID | Name | City | Country | 
|---|---|---|---|
| 1 | Tom Kurkutis | New York | USA | 
| 2 | Ana Fernandez | London | UK | 
| 3 | Antonio Indigo | Berlin | Germany | 
| 4 | Ben | Delhi | India | 
| 5 | Andrew Tumota | Miami | USA | 
Updating all the entries
 Example    
UPDATE Developers
SET Name='Ben';
The result
| ID | Name | City | Country | 
|---|---|---|---|
| 1 | Ben | New York | USA | 
| 2 | Ben | London | UK | 
| 3 | Ben | Berlin | Germany | 
| 4 | Ben | Delhi | India | 
| 5 | Ben | Miami | USA |