To connect to a windows instance in Google Cloud:
Step 1) Go to your project:
Step 2) Click on RDP Button drop down to set new RDP password:
Step 3) Click create or reset windows password:
Step 4) Enter User name ( admin or administrator or any other name)
Step 5) Click Set to get password( this is the password required to put in RDP connection)
Step 6) Click RDP to download RDP file and enter username and password as per Step 4 and Step 5 to connect to your machine:
Step 7) Connect to your instance and you shall be able to have complete control of your machine with admin rights.