Okay, so the quotes after start is where the file location for this is. For example, mine is "E:\pixarkserver\SteamCMD" then you continue with the "/steamcmd.exe" so for me, the line is E:\pixarkserver\SteamCMD/steamcmd.exe. Then, to my understanding, you delete the quotes around '"C:\pixarkserver"'. The part about "+login username password" means you instead put your steam id and steam password instead of "username password" respectively.
for example, if my steam user name was MAGATrump and my pass was 1234 the line would read:
start E:\pixarkserver\SteamCMD/ steamcmd .exe +login MAGATump 1234 +force_install_dir
E:\pixarkserver +app_update 824360 validate +quit
mine is "E:pixarkserver" because I install on my E drive.
You save update.bat in your steamcmd folder
I hope this helped
Edit: You should check out the video posted above, it's probably more help than I am.