How to rename label within a metric in Prometheus

you can use the label_replace function in promQL, but it also add the label, don’t replace it

label_replace(
  <vector_expr>, "<desired_label>", "$1", "<existing_label>", "(.+)"
)

label_replace(
node_systemd_unit_state{instance="server-01",job="node-exporters",name="kubelet.service",state="active"},
"unit_name","$1","name", "(.+)"
)

So, to avoid the repetition you can add:

sum(label_replace(
    node_systemd_unit_state{instance="server-01",job="node-exporters",name="kubelet.service",state="active"},
    "unit_name","$1","name", "(.+)"
    )
)by(unit_name)