serverless/docs/guides/variables/env-vars.md

Read this on the main serverless docs site

Reference Environment Variables

To reference environment variables, use the ${env:SOME_VAR} syntax in your serverless.yml configuration file.

Note:

Keep in mind that sensitive information which is provided through environment variables can be written into less protected or publicly accessible build logs, CloudFormation templates, et cetera.

service: new-service
provider: aws
functions:
  hello:
    name: ${env:FUNC_PREFIX}-hello
    handler: handler.hello
  world:
    name: ${env:FUNC_PREFIX}-world
    handler: handler.world

In the above example you're dynamically adding a prefix to the function names by referencing the FUNC_PREFIX env var. So you can easily change that prefix for all functions by changing the FUNC_PREFIX env var.