Mostly it comes down to the processor. Broadly, Intel 8th generation and newer and AMD Ryzen 2000 and newer can upgrade, and anything older cannot, whatever else is in the machine. The other common answer is that the computer is perfectly capable but has TPM or Secure Boot switched off in its settings, which looks like a failure and is not one. Answer what you know below and this will tell you which of those you are, and what to do next either way.
A browser cannot read your computer's insides.
No web page can see your processor model, whether you have TPM 2.0, or whether Secure Boot is on. Anything claiming to scan your PC from a website is guessing. This is a guided self-check based on what you can see, and it will tell you where to look. For a direct reading, Microsoft's own free PC Health Check app does it properly.
To check: Settings → System → About, and read the Processor line. Or press the Windows
key and R together, type msinfo32 and press Enter.
In "Intel Core i5-8250U" the 8 at the start of 8250 is the generation.
No, and any page claiming it can is guessing. A web browser is deliberately not allowed to read your processor model, whether you have TPM 2.0, or whether Secure Boot is switched on, because that would be a privacy hole. So this is a guided self-report: you answer what you can see, and it tells you which of the four likely answers you fall into and exactly what to check next. Microsoft's own PC Health Check app is the tool that can read those things directly.
On Windows 10, open Settings, then System, then About, and look at the line marked Processor. You can also right-click This PC and choose Properties, or press the Windows key and R together, type msinfo32 and press Enter. You are looking for something like "Intel Core i5-8250U" or "AMD Ryzen 5 3600". The number straight after the dash is what matters.
Because Microsoft drew the line there and has not moved it. Roughly, Intel 8th generation and newer, and AMD Ryzen 2000 and newer, are supported. Anything older is not, regardless of how much memory it has or how fast it feels. That is the one requirement you cannot fix with a setting or a cheap part, which is why a machine from around 2017 or earlier usually cannot be upgraded however healthy it is.
Usually not, and this is the single most common false alarm. Most computers sold from about 2016 onwards have TPM 2.0 built in but switched off in the BIOS, and Secure Boot is often disabled too. Both can be turned on, and the machine then passes. It is worth checking before writing off a perfectly good computer, and it is a ten-minute job rather than a repair.
It keeps working. Nothing switches off, your files stay put and your programs still run. What stops is the flow of security patches, unless you are enrolled in Extended Security Updates, and over time that gap is what matters: new flaws get found and no longer get fixed on your machine. Banking and email on an unpatched computer is the risk, not the computer suddenly refusing to start.
For a machine used mainly for browsing, email, video and documents, genuinely yes, and it will often feel faster than the Windows it replaced. It is a poor fit if you depend on specific Windows software, on Microsoft Office rather than a substitute, or on a particular printer or scanner with Windows-only drivers. Worth trying from a USB stick first, which changes nothing on the machine until you decide.
No. Windows 11 will not fix a slow computer, and on a machine that is already struggling it usually feels slightly worse rather than better. Slowness is nearly always an old hard drive that wants replacing with an SSD, not enough memory, or years of accumulated software. Sort that first and then decide about the version of Windows.
No. Nothing is sent anywhere and nothing is saved. The whole thing runs in your browser from the boxes you tick, and closing the tab is the end of it.
Related: the repair or replace calculator if the real question is whether the machine is worth keeping, why is my laptop so slow if the upgrade is not really what you are after, and buying a new laptop if you have decided to replace it.