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The aim that I am trying to achieve:

Update a value in MongoDB based on its current value.

After googling I have found that $cond operator potentially allows me to solve the problem, so I've written the following query:

db.getCollection('product').update({_id: ObjectId("77b2a57556a5e634d57d9977")}, 
{"$set": {"availability" : 
   {$cond: [ { "$availability": { $eq: true } }, "Yes", "No" ] }}});
but for some reason, it doesn't work and MongoDB throws an exception.
So could you please advise: Is it technically the right approach to use $cond within the update query?
If yes, why the query doesn't work, semantically it looks good to me,
or possibly some other options on how to solve the problem available, please suggest.
So it seems like you are trying to use Update with Aggregation pipeline which is supported in MongoDB >=4.2. And also if your DB version is >=4.2 the correct syntax would be
db.getCollection("product").update(
  { _id: ObjectId("77b2a57556a5e634d57d9977") },
      $set: {
        availability: {
          $cond: [
              $eq: ["$availability", true],
            "Yes",
            "No",
        
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