RAM and CPU is changed moving from a Computing offer to another
To proced with the upgrade / downgrade first you need to stop the instance
then click on the + icon as shown into the image below 
Choose the computing offering of your choice
Please keep in mind that you need to choose between the same offering. i.e. if you have a BASIC xxx instance you can only go to another BASIC yyy computing offer, on the other hand if you have a SSD xxx instance choose another SSD yyy plan.
