How to stop hiss on computer microphone?

Looking at the spec on Amazon & the complaints about this mic, I think you should send it back.

It's designed to look like a cheap low impedance large diaphragm condenser, but in actual fact is an even cheaper high impedance electret condenser [that's the kind of thing you get in a headset].
It needs plugin power of 1.5v from the Mac, just like a headset mic.

On to the bit they hid right at the bottom of the page… S/N [signal to noise] ratio "More than 36dB"!!
I should really hope so… that is possibly the worst S/N figure I have ever seen on a piece of audio equipment. You would hope for 60dB even on a poor mic.

The hiss you are hearing is very likely, based on that figure, being actually generated by the mic itself, so there is nothing you can do to fix it.

There's a good, easy-to-read article explaining signal to noise ratio at MyNewMicrophone - What Is A Good Signal-To-Noise Ratio For A Microphone?

If you want to replace this mic with something better but still as cheap,then maybe such as a Neewer NW700* [this mic comes in many guises with many names & numbers but it's the same thing underneath] for about the same price - but you must get a USB microphone pre-amp & a new XLR-XLR cable before you can successfully plug it into a computer. [They always fail to tell you this in the instructions].
You can get it packaged with a phantom power supply… but you still need to be able to get it into the computer cleanly, so avoid those & get a dedicated USB preamp.

I see that they now make a straight to USB version of this. I have no experience with this one, but it ought to work in theory, as the USB can provide the power the mic needs. Sight unseen, I'd be tempted to go for this type.

*I bought one of these a few years ago just so I could answer the myriad questions we see on here & on our sister site, Sound Design about it & similar mics. I'd hardly swap it for my £2,500 Neumann U87, but to be honest… it's not terrible.