change EC2 parameters (cores, RAM) after I have installed a GUI and some softwares

You can definitely increase/decrease CPU and RAM for your EC2 instance. This can be achieved by change the instance type and switching to the bigger or smaller type depending on the requirement in the same instance family.

If your instance is EBS backed i.e it uses EBS volume for the root disk then follow instructions at: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-resize.html#change-instance-type-of-ebs-backed-instance

To change the instance type of instance store instance: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/resize-instance-store-backed-instance.html

If you want to change to the in-compatible instance type then please check: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-resize.html#migrate-instance-configuration

On EC2, you can't technically change the resources allocated to an instance, but you can easily stop it, change the instance type and start again.

Details here https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-resize.html and the end result is the same than increasing resources, except this can't be done live.