George Barbaz is right, his answer should work just fine.
How ever, here is a maybe more detailed explanation, because you stated you are new to the linux world:
Open the files application and navigate to the folder, where the file you've downloaded is stored.
Right-click and click on 'Open in' ➝ 'Terminal'
Type in the following command.
sudo apt install -y ./nameofthefile.deb
apt is the package manager used by ubuntu-based linux distributions like elementary, the
install keyword explains itself, I guess, and the
-y flag skips the confirmation dialog, you are normally prompted with and approves the actions taken directly.
. is a substitution of the current directory that you are in.
apt needs this to understand, that you want to install a local file and not one from a repository.
That's pretty much everything sumed up, I guess. Makes it maybe a bit more understandable for you.