You cannot change the network adapter type on an existing network adapter. The only time you can choose the network adapter type is when creating a new VM or when adding a new network adapter to an existing VM.

The steps to choose a network adapter while creating a new VM are as follows:

1. Open vSphere Web Client.
2. Log in to your vCenter Server.
3. Click on your cluster and select New Virtual Machine | New Virtual Machine.

4. Go through the steps and hold on to the Customize hardware step. Here you need to choose how many network adapters you need, which port group you want them to connect to, and the adapter type.

To choose an adapter type while adding a new network interface to an existing VM, follow these steps:

1. Open vSphere Web Client.
2. Log in to your vCenter Server.
3. Navigate to VMs and Templates on your home screen.
4. Select an existing VM where you want to add a new network adapter, then right-click on it and select Edit Settings.
5. Select Network from the New device drop-down menu at the bottom, then click on the Add button.
6. Expand the New Network section at the bottom.
7. Select the port group and Adapter Type.

8. Click on OK to finish.