Data frame string matching from one to another and replace with original value

Question

i have two data frame , A and B . In A Data frame two column value and filed . and in B data frame have also value and filed column. i want to match the 'value' column of B to A of'Filed' column, replace the Filed of A to value of B.

A=

 Value                     Filed        

valid username            username
valid username           input_txtuserid
valid username           name
Password                 input_txtpassword
Password                 txtPassword
Login                     input_submit_log_in
Login                     loginid
LOG IN                    SIGNIN

B=

 Value                     Filed        

input_txtuserid           "JOHN"
input_txtpassword          "78945"
input_submit_log_in        "Sucessfully"
City                       "London"
PLACE                      "4-A avenue Street"
PHONE                      789456

I want my data frame to look like this:

C=

 Value                     Filed        

valid username            "JOHN"
Password                   "78945"
Login                       "Sucessfully"
City                       "London"
PLACE                      "4-A avenue Street"
PHONE                      789456