Configuring Elastic Beanstalk Default Instance

Under Instance types, add types you want, and make sure put the desired type in order, for example, the default is t2.micro, t2.small, if you want to make t2.small as default instance type, you need to change the order to t2.small, t2.micro. Once you made the change, you can click Apply, and in the next page click Confirm.

Otherwise, You can go to your auto scaling groups in the ec2 web console, and select the auto scaling group associated with your Beanstalk environment and click edit in the drop down menu. Once this page opens up, there should be a section called Instance Type Requirements. You can click on the "Override Launch Template" under Instance Type Requirements and select "Manually add instance types". This should show the primary instance type which is currently in use and the additional instance types which you can add or remove. Launch template will only show the primary instance type that the is instance is launched with.

Regarding changing the order in the Beanstalk console, I would recommend going into auto scaling and following the steps mentioned above since this is a more accurate depiction then what the Beanstalk console shows you although you are able to select different instance types.