The AWS Documentation for CloudFormation Default values says:
A value of the appropriate type for the template to use if no value is specified when a stack is created. If you define constraints for the parameter, you must specify a value that adheres to those constraints.
This behaviour can be very pernicious, as default values are only set on stack creation and when no value is specified.
Meaning: You cannot update the value by setting a new default value!
Instead, set values by passing them in as parameters in a samconfig.toml file or by passing them in as parameters when you deploy the stack.