Open Audacity and click FILE, IMPORT..., to Choose which audio you wish to edit.

With your cursor selection the portion of audio that you wish to Amplify ,and make sure you have the Selection Tool as your choose while editing( See Picture)

Go to the top bar and click EFFECT, AMPLIFY. This will Amp up or De-Amp the audio you have selected. Also if you mess up or want to select a different portion just click the Undo button.

After clicking EFFECT, AMPLIFY Audacity will bring you to a little pop up screen, which is where you select how much you want to amp up your audio or amp down your audio. Play around with the settings, and just preview to check out what you have before hitting OK to finalize your selection. Also, Click the little box to Allow Clipping.

After finalizing your amp selections your waveform should look slightly bigger, depending on how much you choose to amp up or down.

Once you are satisfied with your editing don't forget to Save your work. In Audacity to save and finalized a project go to FILE, EXPORT... Then simply choose where you want to place your editing audio file.