find_one_and_replace() method search one document if finds then replaces with the given second parameter in MongoDb. find_one_and_replace() method is differ from find_one_and_update() with the help of filter it replace the document rather than update the existing document.

Syntax: 

find_one_and_replace(filter, replacement, projection=None, sort=None, return_document=ReturnDocument.BEFORE, session=None, **kwargs)

Parameters 
filter: A query for replacement of a matched document. 

• replacement: replacement document. 
• projection: it is optional.A list of a field that should be returned in the result. 
• sort: key, direction pair for the sort order of query. 
• return_document: ReturnDocument.BEFORE (default) will return the original document without replacement. ReturnDocument.AFTER will return the replaced or inserted document.
  **kwargs: Additional commands. 

Sample database used in all the below examples:

Example 1: 

import pymongo


# establishing connection
# to the database
client = pymongo.MongoClient("mongodb://localhost:27017/")

# Database name
db = client["mydatabase"]

# Collection name
col = db["gfg"]

# replace with the help of
# find_one_and_replace()
col.find_one_and_replace({'coursename': 'SYSTEM DESIGN'},
                         {'coursename': 'PHP'})

# print the document after replacement
for x in col.find({}, {"_id": 0, "coursename": 1, "price": 1}):
    print(x)

Output:

Example 2: 

import pymongo


# establishing connection
# to the database
client = pymongo.MongoClient("mongodb://localhost:27017/")

# Database name
db = client["mydatabase"]

# Collection name
col = db["gfg"]

# replace with the help of
# find_one_and_replace()
col.find_one_and_replace({'price': 9999}, {'price': 19999})

# print the document after replacement
for x in col.find({}, {"_id": 0, "coursename": 1, "price": 1}):
    print(x)

Output: