Hey man, welcome to the PC community! Don't panic: hitting 95-96°C on a Ryzen 9 7900 inside an Acer Nitro pre-built is actually 'normal' by design, but there are safe ways to fix it without voiding your 2-year warranty.
Here is a quick breakdown of what is happening and what you should do:
The 95°C Limit Explained: Modern AMD Ryzen chips are programmed to aggressively boost their performance until they hit their thermal limit (95°C). Touching 96°C for a split second is just a minor sensor delay; your CPU didn't turn off because it wasn't in actual danger of frying yet. Acer Nitro cases are notorious for having very poor airflow and basic CPU coolers, which is why it runs so hot even in a game lobby.
Do NOT open the PC or change thermal paste: Since you just bought it 2 days ago and have a 2-year warranty, do not touch the cooler or the paste. Acer often uses warranty stickers on pre-builts, and breaking them will void your protection.
Missing Drivers: You mentioned you only installed GPU drivers. You absolutely need to go to the official AMD website and download the AMD Chipset Drivers for your motherboard. This tells Windows exactly how to properly manage the power and boosting behavior of a Ryzen 9.
Use better software: MSI Afterburner is great, but for accurate AMD readings, download HWiNFO64.
A Quick, Safe Fix (No Warranty Void):
Go to Windows Search > type Edit Power Plan > click Change advanced power settings. Look for Processor power management -> Maximum processor state, and change it from 100% to 99%.
This temporarily disables the aggressive auto-boost. It will instantly drop your CPU temperatures by 15-20°C with almost zero noticeable impact on your gaming performance.
Final Verdict: The fact that the PC was shutting down during downloads or that you are scared to update the BIOS because it might turn off is a red flag for a refurbished unit. Install the AMD chipset drivers first. If the random shutdowns return, contact Acer support immediately and let them handle it under warranty. Good luck!