How do you open this fluorescent light fixture?

First, Are you entirely sure this is a fluorescent fixture and not an integrated LED fixture? Because it's awfully, awfully thin for a fluorescent. The only tubes I can imagine even having a chance of fitting are T5s (5/8" diameter).

However, fluorescent fixtures never require tools to get the tubes out. IF this was fluorescent - and it's very much my hunch that it is not - you would push upward a little bit (1/8" or so) along one of the long edges (either one) and then slide it toward the edge, making the other side pop out, i.e.

However I wouldn't do a whole lot of pushing. If this doesn't happen easily, the next step is to presume it's a bulbless LED, and search it for a model number or identifying marks, so you can search for that and see if it is.

What's the deal with bulbless LEDs? Isn't that bad? Sure, I get it. We all grew up our whole lives with every type of lightmaker - incandescent, halogen, fluorescent, HID - all having the bulb as the weak link/consumable. So it's hard to believe that with LEDs, the lightmaker is actually the most durable part. But it is silicon electronics - and when's the last time you changed a transistor on a stereo or TV? But vacuum tube TVs and stereos required changing them all the time - to the point where historians/purists had to learn to make their own vacuum tubes!

Now, the weak points are in the electronic power supply that converts AC power into the correct current of DC for the LEDs. Cheap components (notably capacitors), solder (RoHS lead-free solder that crystallizes; many PCB problems these days are cured by running the board through a solder reflow) and other electronics-side stuff - but not the LEDs proper. Those will outlive all of us.

Of course this is cold comfort if your fixture is kaput. Most of us don't want to pull down LED fixtures and carefully identify the constant-current electronic driver and find another one of the same form-factor. So it's down to replace the whole fixture - and learn the lesson that the supplier isn't very good.

Given the way our markets are being flooded with cheap junk (especially Amazon!!), this type of failure is shamefully common. I myself have chosen to stay with real fluorescent, because a GE ballast and Sylvania tubes are safe choices.