I tried changed my username but I think I missed a step, so now my login does not work for updates and installment

Here is the Apple article with instructions for changing the user account name:

https://support.apple.com/en-us/HT201548

You should have created a second admin user the first time which will make it easy for you to fix your mistake. Go through the Apple article step by step to correct the mistake.

To create a new admin user account requires deleting a hidden macOS file so that macOS is tricked into running Setup Assistant again the next time you boot the computer. You need to boot into Recovery Mode using Command + R. Once in Recovery Mode you need to launch the Terminal app from the Utilities menu. Then run the following commands making sure to press the "Return" key at the end of each line to execute the command.

You may first need to disable macOS System Integrity Protection (SIP) which is a security feature meant to protect macOS system files:

csrutil  disable

Now you need to find and confirm the name of the boot volume in case you changed it from the default "Macintosh HD":

mount  |  grep  -i  volumes

The output will show the name of the boot volume (and possibly some other volumes as well), but you should recognize the boot volume if you changed its name.

Now run the following command to delete the hidden macOS file ".AppleSetupDone" making sure to use the name of your boot volume in place of "Macintosh HD" if you changed it from the default):

rm  -i  "/Volumes/Macintosh HD/var/db/.AppleSetupDone"

Now re-enable system security (SIP) with this command:

csrutil  enable

You should now reboot the computer and be greeted with Setup Assistant where you it will allow you to create a new admin user account. Once booted to the new admin user account you can use the instructions I provided previously to fix the original user account.