You can go back; what kind of installation did you do (MSI, ZIP, installer)? You will also have to delete C:\Users\yourUsername\AppData\Local\Arduino15.
I don't think that the change from 2.0.3 to 2.0.4 is the root cause of the problem.
Before doing so, you might want to check e.g. your antivirus; it might block access to the ports by the tools used for the upload; you can configure it to exclude the above directory and see if it helps.
If you use a terminal program (e.g. realterm, putty), can you access the port?
Your topic has been moved to the dedicated IDE 2.0 section of the forum.
Use the Windows Device Manager to determine which COM port appears when you plug the Arduino Uno R3 into the USB port, and select that COM port in the Arduino IDE for uploading.
Hi @emphyx. I'm sorry you had trouble uploading with Arduino IDE 2.0.4.
Please tell us which USB to serial adapter chip your Arduino board has. This is a black chip near the USB socket.
The chip will usually be identified by writing on the top. This might say something like "Atmel MEGA16U2" or "WCH CH340G" or "SILABS CP2102" or "FTDI FT232RL".
If it isn't clear, alternatively you can provide the link to where you bought the board from and we'll see if we can determine the chip from the product listing.