Step 1: Open your Applications folder and Look for "Install macOS Catalina" click it ONCE and look at the information to the right side. If it says 8.24~ GB continue onto step 3 if it says 19mb or you don't have the installer at all continue onto step two

STEP TWO: if you have the 19mb install delete it by dragging it to the trash. Once that is done go to the apple in the top left-hand corner and system preferences

After that go to Software updates 

it will check for updates, you will see 10.15.6 upgrade click upgrade and it will start to download.

Step 3: once you verify you have the full installer you can open it via your applications folder double-clicking it. it will bring you up to the license agreement, click agree and then make sure "Macintosh HD" is selected. if it is grayed out click it and it will tell you how much space is needed. if it is not grayed out click "install"

Step 4: Once you click install you will be prompted for an admin password, type your password and click "OK"

After you click ok it will start to install, please allow up to an hour for the install to complete.