As a business owner, leader or HR manager, what should you do with employees who are not happy with their job?

1- Ask for the Reason.

2- Try to solve the reason.

the reason could be asking for salary increase or he can't full fill the position or not comfortable with manager & colleagues .

Salary issue : check if it's acceptable, and if his work is good enough for it.

Position Issue: transfer him to anther suitable position if he is qualified enough and if not at all he can help in filing and documentation .

Not comfortable with manager & colleagues : Ask the HR department to control this issue by investigation and executing punishment based on the violation matrix.

Finally it's not professional from your side to terminate someone except in3 cases :

1- Fatal mistake.

2- Really not qualified and rejecting any training or orientation.

3- Makes the work environment non-comfortable.

EWI (Early Warning Indicators) and Grievance Redressal Forums to be in place for the HR Manager to take required action to retain good workers leaving (Attrition). Trendline of EWI vs Attrition as well as Grievance raised vs Attrition for last 2 years to be studied with present conditions 

As HR you need to keep the staff happy (within reason of course). So find out from the employee what is making him unhappy and try to resolve it by coming up with different solutions. If the matter can not be resolved, the employee should find employment elsewhere.

I guess in all different roles you have mentioned, the approach would be different as all three have different stakes in the same person if you know what i mean

I will speak for a leader.  i would do this in two ways.  First informally and then formally if required. 

I need to informally, speak to this person.  if possible out of office environment.  Find out the underlying reasons which are affecting his work and he seems to be off.  Is it personal or professional ?  Is he subject to harassment at work or disturbed by a colleague or maybe me myself.  I have had people not happy with me too.  Once i know the reason, even if it is of a personal nature, i would 100% try to help if i want him to perform.  i once found a person who was in the wrong job which is why he was not happy.  i found him a job he was happy with.  Its as simple as that.  I can always find someone to take his place.  The right person i mean.

Mostly the formal route is not required.  I never required until now.  

• Try to determine the reasons and find the appropriate solutions
• Provide psychological comfort
• Enabling the employee to save energy and time
• Improving employee relations
• Giving more meaning to the employee's professional screw
• Allow the employee to discover his or her strengths
• Intensify communication flows
• Employee Rewarding Consideration
• Be more attentive
• Listen to the workers, listen to their opinions, their testimonials and their suggestions
• Be more receptive
• Motivate
• Delegating Responsibilities
• Composition of small groups
• Creating competition
• Offering  the initiative
• Identify potential problems
• Promoting teamwork
• The training

First of all become a good listner for them. It might be possible that employees are not happy becaues there is no one to listen to their challenges. Try to resolve those challenges. It may be official challenges but you can always provide a solution with personal touch which will be most beneficial & can give you best results.

Thanks 

I strongly support my colleague Mr. Rabahi's answer