Installing a .deb file from the terminal on Ubuntu?

No terminal? Right click the .deb and open it with Software Install (App Center on newer Ubuntu), does the same thing. To remove it later use the package name, not the file name: sudo apt remove packagename. If you don't know it, dpkg -I file.deb shows it.

sudo apt install ./yourfile.deb The ./ is the bit people miss, without it apt goes looking for a package by that name in the repos and fails. Doing it through apt instead of dpkg -i means it pulls in any missing dependencies for you.