It depends a bit on your installation. There are basically 2 options:
1) start with a clean install
2) upgrade your current installation
There are pros and cons to each approach. The first is more work since you start with a clean image, install domoticz from scratch and then put back your domoticz existing installation (including database but also any script you may have built). The second is usually 'easier' but can lead to upgrade problems. The raspberry pi foundation does not recommend it but many people have done it before and it works 99.99% of the time (unless you have done some heavy customisations that are incompatible with the new release). I have done multiple of these without any issue but sometimes (after many years and 2 release upgrades), it's sometimes better to start clean.
Since you consider yourself a beginner, does that mean that you have not done many system customisations (install incremental packages and libraries)? Do you have scripts (lua, dzvents, python) and are they stored on the system or in the database (using the domoticz UI)?
Anything else you can tell us before making a recommendation (what interfaces you have, devices you use, plugins you installed)? What hardware do you run on to begin with (probably a Pi?)?
Step 1 is always to make a complete backup of your system. If you run domoticz using a micro SD card, make a complete copy of the micro SD card before doing anything!
If you want to just upgrade rather than complete reinstall, google can be your friend. Some examples:
https://pimylifeup.com/upgrade-raspbian ... an-buster/
https://www.cyberciti.biz/faq/update-up ... 10-buster/
https://raspberrypi.tilburgs.com/upgrad ... (in dutch)