$ jq '.fruits|del(.count,.open)|with_entries(.value |= .price)' fruits.txt
{
  "apple": 2,
  "banana": 3
}

# with_entries(f) is an alias for to_entries | map(x) | from_entries

jq '.fruits|del(.count,.open)|to_entries|map(.value |= .price)|from_entries' f
{
  "apple": 2,
  "banana": 3
}

$ jq '.fruits|del(.count,.open)|[to_entries[]|{(.key): .value.price}]|add' fruits.txt
{
  "apple": 2,
  "banana": 3
}