The first thing I would do is turn up temp control on gas valve.Control is located on front of gas valve at the bottom front of heater.They usually have a indicator dial on front with an arrow pointing direction.Turn dial to higher setting when burner is not on and this should turn burner on and provide hotter water. The control is normally turned clockwise to increase temperature.If control is already turned all the way up then water heater may need to be replaced.Depending on water quality in your area heater may have a large amount of mineral build up and it may not be capable of producing enough or hot enough water. If this the case all I can suggest is to try to drain tank and flush some of the deposits out. If you have installation manual it will give you instructions on how to do this. If not see if you have working shut off valve on cold water line ,if it works shut it off and connect drain hose to drain valve on bottom right side of gas valve. Put hose to floor drain or sump basin if you have one. Turn control knob on top of gas valve from on to pilot,this will leave pilot light on but prevent burner from working while draining. Open drain valve ,flow rate may be slow or not at all. Open hot water side of some faucet to provide air to help draining. You may try opening water supply valve to heater a few times to aid in draining. If you can get it to drain,shut drain valve off when done,turn cold water supply back on and shut off hot water outlet. Then when tank is full open hot water vavle to bleed air out of heater and piping. Turn control knob on top of gas valve back to on position. This should bring burner back on .It will take about 1/2 hour to heat. If you can't get it to drain or this doesn't help you might consider getting a new 40 gallon heater. Hope this helps. Thanks