I wonder why I could not change the spindle speed during my 1st gcode run but I was able to change the feed rate. Mach3 is set to control the spindle speed and in trial runs using M3 S*********x or M5 works well

As my cut was progressing and since my speeds were set low, I click the plus sign in the Feed Rate window and the feed rate increased but in the Spindle Speed window clicking the plus sign nothing changed.

Perhaps I'm doing something wrong?

The spindle override should work fine as long as Mach is in full control of your spindle.
Does the % increase when you increase the override? If it does then either Mach is not in control of your spindle properly or your spindle is already at Max according to your Spindle Pulleys/Motor Tuning.
Hood

Yes the % increases and my pulley setting is at Max 24K. But after checking all settings in Mastercam I think that the postprocessor is the problem.

I downloaded the postprocessor from the Mach3 site (MasterCam_MACH3B.pst) and looking at the code I noticed this line at the top:

# Revision log:
# --------------------------------------------------------------------------
# Programmers Note:
# CK  12/07/05  -  MODIFIED MACH2 POST
#                  -Changed Max/Min spindle speeds to 2000/320RPM

My gcode has always S2000 so it must be the PST but I don’t want to venture editing the PST since I never done it. Should I open another thread and ask for help on how to edit the PST?

You would be best talking to MasterCAM about editing the PP as unless someone here has MasterCAM and has edited a PP then you will be unlikely to get an answer.
Hood

Thank you Hood, I will take care of it

I opened the Mach3B post processor in Notepad and looking at the lines I found this:

max_speed   : 2000  #Maximum spindle speed
min_speed   : 320   #Minimum spindle speed

Changed the 2000 to 24000, saved the file, and the problem is now solved, can put any speed up to 24000 when I select my tool and the gcode is correct

The above is for other members if they have similar problems but be careful and first make a backup copy of the pst file and make sure you don’t change anything you are not sure. Also try a few simple operations to assure what you did works well