jq ' map(if .key | contains("label") then .value="kuku" else . end) '

cat data/ann.json | jq '.metadata.annotations | to_entries ' cat data/ann.json | jq '.metadata.annotations | to_entries | map(if .key | contains("label") then .value="kuku" else . end) ' 
[
  {
    "key": "deployment.kubernetes.io/revision",
    "value": "1"
  },
  {
    "key": "machine.openshift.io/owned",
    "value": ""
  },
  {
    "key": "label1",
    "value": "stam"
  },
  {
    "key": "label2",
    "value": "another"
  },
  {
    "key": "label3",
    "value": "one"
  }
]
    
[
  {
    "key": "deployment.kubernetes.io/revision",
    "value": "1"
  },
  {
    "key": "machine.openshift.io/owned",
    "value": ""
  },
  {
    "key": "label1",
    "value": "kuku"
  },
  {
    "key": "label2",
    "value": "kuku"
  },
  {
    "key": "label3",
    "value": "kuku"
  }
]