How do I un mod a galaxy Tab A

Here is how you do it:

1. Back up your data, obvs.
2. Put the tablet into download mode. Ensure that it is off, then press and hold the power button, the home button, and the volume down button at the same time. Use the volume up button to accept the warning that appears. In the upper right corner it will say PRODUCT NAME. Take note of that for the next step.
3. Go to SamFw, enter the model number, and download the appropriate firmware image for your locale. Extract it somewhere.
4. Download and install Samsung USB Driver.
5. Download and extract Odin.
6. Run Odin and load firmware files from SamFw. The filenames in the firmware package start with the name of the slot they belong in in Odin. If your tablet doesn't have cellular capability then there will be no CP file. Your firmware package will come with a CSC file and a HOME_CSC file. The former will wipe user data clean and the latter will preserve it. It's highly recommended to use the CSC file however, especially when reverting to stock. Don't change anything in the Options or Pit tabs.
7. Connect the tablet to the PC with a USB cable. Odin should say Added!! in the console. Click Start to begin flashing

Note, there is one important caveat. If Samsung devices detect a non-Samsung bootloader, they will trip the Knox WARRANTY VOID bit and Knox-related features will no longer work, such as Knox Workspace, Samsung Pay, and Secure Folder. This is irreversible because it blows an eFuse inside the SoC. The only way to fix this is to replace the motherboard. Check the status of the warranty bit in the tablet's download mode. If you don't care about Knox then you can use it as normal, if it is 0 then it is still intact and you didn't actually install a custom bootloader (and therefore a custom ROM).

Also please note that I am not responsible for you bricking your device.