Existing Cloudwatch log group retention policy

we are using TFE and have created several lambdas, which in turn created several log groups.
we would like to modify the retention policy on these log groups. what is the best way to modify existing log groups with TFE? that were not created by TFE but by the lambdas when they were first run.

You can create the cloudwatch log group for lambdas with Terraform. The expected name of the log group for a lambda is /aws/lambda/{lambda-function-name}.
For existing log groups, you can just add the cloudwatch log group resource to your terraform config with suitable log retention, import log group to terraform and run terraform apply.