You can add a new key-value pair to an existing dictionary in Python using the assignment operator (=) or the update() method. Here are five examples demonstrating both methods:

1. Using the assignment operator:

# Existing dictionary
my_dict = {'a': 1, 'b': 2}

# Adding a new key-value pair
my_dict['c'] = 3

print(my_dict)
# Output: {'a': 1, 'b': 2, 'c': 3}

2. Using the update() method:

# Existing dictionary
my_dict = {'a': 1, 'b': 2}

# Adding a new key-value pair
my_dict.update({'c': 3})

print(my_dict)
# Output: {'a': 1, 'b': 2, 'c': 3}

3. Using the update() method with keyword arguments:

# Existing dictionary
my_dict = {'a': 1, 'b': 2}

# Adding a new key-value pair
my_dict.update(c=3)

print(my_dict)
# Output: {'a': 1, 'b': 2, 'c': 3}

4. Using the dict() constructor with keyword arguments:

# Existing dictionary
my_dict = {'a': 1, 'b': 2}

# Adding a new key-value pair
my_dict = dict(my_dict, c=3)

print(my_dict)
# Output: {'a': 1, 'b': 2, 'c': 3}

5. Using a dictionary comprehension:

# Existing dictionary
my_dict = {'a': 1, 'b': 2}

# Adding a new key-value pair
new_key, new_value = 'c', 3
my_dict = {**my_dict, new_key: new_value}

print(my_dict)
# Output: {'a': 1, 'b': 2, 'c': 3}

All these methods will add a new key-value pair to the existing dictionary.