When I am UPDATING my Electrum wallet, do I need to do anything like delete the old? I have not done this before, only downloaded my first ever wallet.
I will do it for Windows. Does it automatically go onto the default wallet once installed or do you have to start from scratch using your seed?
As long as the initial installation was in default location and you haven't made any changes in that, the new installation will simply replace the old files with the new ones and you will have the new version running.
As for the wallet, wallet files are stored separately in your %APPDATA% folder and they are not touched when upgrading. There is also another file called config which contains your client settings, last wallet created and more. It will open the last wallet file you opened.
However, it is best to always have a backup of your seed phrase to be able to recover your funds in case of a catastrophe (hardware damaged) and also when upgrading the major versions (eg. from 3.x.x to 4.x.x) sometimes there are major changes that may also modify the wallet file. Having a backup of the seed phrase could be useful in case something went wrong during that conversion.