Please see answers below.
Is it possible to stop/pause the instance without terminating to keep the data? --> No feature in place yet to pause an instance. To keep the data you can use the persistent filesystem feature (https://lambdalabs.com/blog/persistent-storage-beta)
Is it possible to modify the number/type of GPUs attached to the instance? --> No feature in place as well to modify number type of GPU attached to a running instance.