Basically you can't really. The hiss is due to the recording being made too low to begin with. Not enough volume and you get hiss caused by the electronic circuitry. You can cut the high frequencies maybe if the mixer has the ability yo do that or you may find a noise reduction plugin in the effects section but either will reduce the quality of the audio.
. . . . How do i increase the volume of the video (speaker) but decrease the hissing background noise? . . . .
To add the @CubeAce comment - working in timeline mode:
Adjust the volume of the audio so you can hear the speech reasonably well, then right click the video and select Audio and waveform display
From the slide out menu select Sound optimisation.
You will now see the tools for various audio functionsfunctions
Use the Noise reduction De-Hisser tool to minimise the background noise, then the Equalizer to emphasise the speech.
You may have to go back and forth between these tools to get the best balance of adjustments.
Note - over doing the 2 tools adjustments can introduce warbling and other odd audio effects, and may not reduce the background hiss sufficiently. Hiss is 'white noise' and can be difficult to remove.