In order to change an instances public IP, you will need to follow these steps:

1. In the Google console edit the instance you want to change the public IP for. Take note of the Zone the instance is deployed in

2. Scroll down to the Network interfaces area and edit the nic0: interface


3. Under the External IP section take note of the ip name. After writing down the ip name, click on the drop down and select Ephemeral and then Done

4. Scroll to the bottom of the page and click on Save


5. In the Google console go to VPC network and select External IP addresses

6. Select the IP which you just changed from static to Ephemeral and click on Release Static Address at the top of the page. If the IP was correctly changed on the instance to Ephemeral you should see a warning in the In use by column with None

7. Once the IP has been release, in the same page click on Reserve Static Address 

8. Type in the same name as the old staic IP, select the region you took note of in step 1 and attach the IP to the instance you set the Ephemeral IP for in steps 1-4 and click Reserve. 

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