I have a SR Suntour NEX (42x34x24) crankset and got my system really bad recently. But as I've used the second (34) chainring most of all I would like to replace only this one. Is it okay? Does my crank allow it?
The Suntour NEX cranksets are marked as "Interchangable Chainring Design" by Suntour, while the XCM's are marked as "Full Interchangable Chainring Design".
This means that you can replace the big ring on a NEX crankset, but the middle and small ring are one piece. So, if you want to replace the middle chainring, you have to buy a whole new crankset (or somehow source the middle and small together). Given the NEX is a fairly low end part [~ 30 USD], another crankset is likely a better idea [ You can pick up a Shimano crankset or similar with all chainrings replaceable for around 50 USD ]. Note that a new chainring would have been likely ~20 USD if you could have replaced it.
Also note that you can go a good while with a "worn" crankset provided the teeth aren't mucked up too badly (e.g. bent), so "suck it up and deal with it" may be a good option depending on what state the crankset is in.
The crankset replacement procedure is here - you may need some tools you don't own.