How do I set a VPC in AWS to 'default VPC' = true?

Unfortunately this is not currently possible:

The introductory blog post Amazon EC2 Update - Virtual Private Clouds for Everyone! outlines in section Default VPCs for (Almost) Everyone that "The default VPC features are available to new AWS customers and existing customers launching instances in a Region for the first time".

Furthermore this is indirectly confirmed by the information within Your Default VPC and Subnets, see Deleting Your Default VPC in particular:

You can delete your default VPC just as you can delete any other VPC. However, after you've deleted your default VPC, it's gone. Now, you must specify a subnet in another VPC when you launch an EC2 instance, because you can't launch instances into EC2-Classic. [...]

If you delete your default VPC and then need to restore it, you can contact AWS Support to have your account reset so that we can create a new default VPC for you.

[emphasis mine]

I recall reading somewhere that they might add respective self-service it later on, but can't find it again and vague announcements like this are rather unusual for AWS anyway, so I might be wrong (and wouldn't hold my breath either way).

As of June 2018 (might have been available earlier) you can create a (new) default VPC by using Actions --> Create Default VPC