Non backlit to backlit

It's possible if both conditions are met:

1.  Check if motherboard has both connectors, one for keyboard and one for keyboard light.

2.  Check if the replacement cost is feasible as some keyboards are built-in with the palm rest as a whole unit.

It should be possible, since both keyboard types are offered.  However:

1.  You can't replace just the keyboard -- you must replace the entire palmrest assembly.  On a system this new, that means you'll need to see if Dell sells the part;  they may not -- and if they do, it'll be more expensive than you might think.

2.  You must strip the entire system down to change the keyboard-palmrest assembly.  It's not a trivial procedure and any damage done in the process will not be covered by warranty.

Service manual is here:

https://www.dell.com/support/home/en-us/product-support/product/inspiron-15-3520-laptop/docs

I want to upgrade my Dell Inspiron 15 3520 keyboard. By default it has non backlit but i want to attach a backlit keyboard is it possible?

Answered above.  It is possible, but requires a complete system teardown.  Service manual above.
The keyboard itself is part of the palmrest assembly, to which most of the rest of the system is attached -- only you can evaluate whether it's worth the effort (or cost, figure on a fairly hefty labor bill to replace the part).

The part isn't expensive.

https://www.parts-people.com/index.php?action=search&search=inspiron+15+3520+palmrest