There's at least two ways.

Right click the file scilexer.dll in your notepad folder and select properties. Then verify the certificate manually.

Another way is to download the same version from the notepad++ website and compare your version with theirs.

Really I wouldn't worry so much as long as you're not a CIA target they have no incentive to spent the time to deploy such a DLL on your system.

Update and run npp? Maybe copy your current dll to make sure it's not overwritten.