Fans are running on high speed need to clean the inside of my desk top

iMacs are sealed units that cannot be opened up by the user without special tools and experience The best you can do is apply compressed air to the vents on the bottom of the unit. Unless, of course, you are one of those types that loves to take things apart.

However, runaway fans are often not caused by accumulated dirt but by corruption/glitches in the SMC system (System Management Controller)

How to reset the SMC of your Mac

I have an issue with using "canned air" on the vents. It can turn "easily-removed" lint to "impossible-to-access" lint by blowing it deeper into the computer. If you shake the can, you can form moisture inside the computer. That would be bad.

I use a strong vacuum cleaner and a non-static pasty brush (wood handle; natural, not synthetic bristles). I hold the vacuum nozzle about 2 inches (50cm) from the bottom vents and use the brush to flick the dust on the vents taward the nozzle. This may be easier to do with the iMac face down on a flat soft surface. Just make sure that it is stable on the surface. If not, get help to steady it.

As already pointed out, every iMac starting with the Late 2012 models has a sealed case and only professional disassembly is recommended.

iMacs have been available for over 20 years in hundreds of configurations. We cannot see your iMac, cannot read your mind and cannot help you unless you carefully read Writing an effective Apple Support Communities Question and then write an effective question.

Until then reset the SMC 3-4 times back to back

[Edited by Moderator]

+1