How do I switch back from Sierra to Mavericks?

Upgrading can be a bit taxing on the system which could explain some of your problem here. In addition often a system running hot also likely has a hardware level problem!

The first thing I would do here is try to address the hardware level issues and then if they fail we can revert the OS back but in doing so you may encounter other issues. Sadly, damed either way ;-{

Lets quickly review what you should have done before attempting the upgrade (not to say you didn't do this):

• Clean out as much junk as you can making sure you have a fair amount of free space (1/4 of the drive is a good amount)
• Clean out the old cache & log files, here's a good tool to do that: Disk Doctor
• Make a TimeMachine backup (don't clone!)
• Create a bootable OS installer USB thumb drive follow this guide: How to create a bootable macOS Sierra installer drive
• Boot up under an external drive (or the installer you just created) too allow you to run Disk Utility on your internal drive to make sure the drive is in good shape.
• If the hard drive (not a SSD) has never been defragmented it's a good time to do that! Here's the tool I use: Drive Genius
• If you can, I would also open the system up and clean out any dust build up using a soft brush and can'ed air

Thats the basic steps I go though for every system I upgrade, sure you can jump over a few of these but often it comes back to bite you.

Some other things to consider...

If you upgraded your HD to a newer drive make sure its SATA speed matches what your system can support. Often I find people put in the too fast a drive in (or one that can't auto sense the systems speed).

The HD cables in a few of the systems don't fully support SATA 6.0 Gb/s so you may need to switch it out. in addition some systems cables can wear from the bouncing around they get so you may need to replace the cable and make sure the case pad is not broken near to drive bay.

I have yet to find a need to roll back an OS upgrade that wasn't a hardware level issue when I have a system running hot.

OK, I don't care! I just want my dam old OS back!!! OK, OK I hear you ;-}

To start with you need the older OS installer! As you'll need to create a bootable USB thumb drive installer with the older OS on it. Then you just boot up under it and run the OS installer! Here's the instructions to do this: How to create a bootable OS X 10.9 Mavericks USB drive

BUT!! Things could get messy here... You may find your system is still running hot & you may loose access to some of your programs that are still on your system. You likely need to completely wipe the drive down to get things working. Make sure your back up is good!!

So why is the system running so hot to begin with??

• The state of the drive may not have been ideal, fragmentation is often an issue. The space the older OS sat is not large enough for the new OS (this happens over time as updates are added).
• The file indexer process need to rebuild the index of all of your files as Mavericks index is not compatible with the new spotlight services so if you have a lot of files it can take sometime.
• Lastly, any issues with the drive SATA cable can create a CRC storm which builds up into a massive problem! This is the symptom of a cable and/or drive failure.