Which one of the following types of statements is an instruction to replace the existing value of a variable with another value?

The correct answer is the assignment statement. An assignment statement is specifically designed to update the value of a variable by replacing its current value with a new one. When this statement is executed, it takes the value on the right side of the assignment operator and stores it in the variable identified on the left side, effectively changing the variable's contents to the new value.

In programming, an assignment typically involves the use of an equals sign, such as variable = newValue;. This operation is key to manipulating data within a program, allowing for dynamic changes in state as the program executes.

The other options represent different concepts: a declaration introduces a variable and specifies its type without assigning a value, initialization refers to the first assignment of a value to a variable typically at the time of declaration, and an update generally encompasses broader changes but is not specifically tied to variable assignments in the same way as an assignment statement.