Yes, with fly scale memory <size-in-megabyes> (docs).
You can, likewise, change the number of VMs (fly scale count <number>), change regions (fly regions add <list-of-region-ids>) where they are deployed to, change vCPUs available to the VM (fly scale vm <cpu-type>) on-the-fly (all of which are idempotent ops: Managing applications with code - #2 by kurt).
Similarly, disk space may be increased too (but never decreased): Increase disk capacity on VM - #2 by ignoramous
Note though, you’d typically need at least 2 or more app instances to avoid downtime when the app is being scaled up or down.