The first version of the code you write may not be of production quality. It is OK to first concentrate on making the code work, rather than worry over the quality of the code, as long as you improve the quality later. This process of improving a program's internal structure in small steps without modifying its external behavior is called refactoring.

• Refactoring is not rewriting: Discarding poorly-written code entirely and re-writing it from scratch is not refactoring because refactoring needs to be done in small steps.
• Refactoring is not bug fixing: By definition, refactoring is different from bug fixing or any other modifications that alter the external behavior (e.g. adding a feature) of the component in concern.

Given below are two common refactorings (more).

if (isSpecialDeal()) {
    total = price * 0.95;
    send();
} else {
    total = price * 0.98;
    send();
}

if (isSpecialDeal()) {
    total = price * 0.95;
} else {
    total = price * 0.98;
}
send();

if is_special_deal:
    total = price * 0.95
    send()
else:
    total = price * 0.98
    send()

if is_special_deal:
    total = price * 0.95
else:
    total = price * 0.98

send()

void printOwing() {
    printBanner();

    // print details
    System.out.println("name:	" + name);
    System.out.println("amount	" + getOutstanding());
}

void printOwing() {
    printBanner();
    printDetails(getOutstanding());
}

void printDetails(double outstanding) {
    System.out.println("name:	" + name);
    System.out.println("amount	" + outstanding);
}

def print_owing():
    print_banner()

    # print details
    print("name:	" + name)
    print("amount	" + get_outstanding())

def print_owing():
    print_banner()
    print_details(get_outstanding())

def print_details(amount):
    print("name:	" + name)
    print("amount	" + amount)

Some IDEs have builtin support for basic refactorings such as automatically renaming a variable/method/class in all places it has been used.

Refactoring, even if done with the aid of an IDE, may still result in regressions. Therefore, each small refactoring should be followed by regression testing.

Given below are some more commonly used refactorings. A more comprehensive list is available at refactoring-catalog.

1. Consolidate Conditional Expression
2. Decompose Conditional
3. Inline Method
4. Remove Double Negative
5. Replace Magic Literal
6. Replace Nested Conditional with Guard Clauses
7. Replace Parameter with Explicit Methods
8. Reverse Conditional
9. Split Loop
10. Split Temporary Variable

One way to identify refactoring opportunities is by code smells.

A code smell is a surface indication that usually corresponds to a deeper problem in the system. First, a smell is by definition something that's quick to spot. Second, smells don't always indicate a problem.
--adapted from https://martinfowler.com/bliki/CodeSmell.html

An example (from the same source as above) is the code smell data class i.e., a class with all data and no behavior. When you encounter the such a class, you can explore if refactoring it to move the corresponding behavior into that class is appropriate. Some more examples:

• Long Method
• Large Class
• Primitive Obsession
• Temporary Field
• Shotgun Surgery

Periodic refactoring is a good way to pay off the technical debt a code base has accumulated.

Software systems are prone to the build up of cruft - deficiencies in internal quality that make it harder than it would ideally be to modify and extend the system further.Technical Debt is a metaphor, coined by Ward Cunningham, that frames how to think about dealing with this cruft, thinking of it like a financial debt. The extra effort that it takes to add new features is the interest paid on the debt.
--https://martinfowler.com/bliki/TechnicalDebt.html

While it is important to refactor frequently so as to avoid the accumulation of ‘messy’ code (aka technical debt), an important question is how much refactoring is too much refactoring? It is too much refactoring when the benefits no longer justify the cost. The costs and the benefits depend on the context. That is why some refactorings are ‘opposites’ of each other (e.g. extract method vs inline method).