Hello, I made an import to edit the "Mortal Kombat 2" screen for the NES, I already edited it but I don't know how to export it to the game now, when I use the "Export -> put CHR into nes file" option, I open the rom and continue with the same screen as before editing. How can I make the new design stay in the rom?

Before:

Import:

Select title:

Image to edit:

After:

edited screen:

There are no changes:

But I have noticed that in the "Tileset" session there are no changes and when exporting it returns the same without changes. What can I do to keep the changes in the rom?

I need help please

The action only refers to CHR, that is, tile/pattern data. Each NES game uses its own convention to store nametable data (the table  that holds which tile goes where on screen) in various ways.
This is outside the scope of NEXXT, which focuses on creating original assets for new games (even if it can be used partially for romhacking too). 

To give some pointers where to go next, i suggest using FCEUX to figure the nametable for the titlescreen out:
1) In fceux, load your game.
2) Go to Menu > Debug > Nametable viewer
3) In Nametable viewer, hover the mouse over some easily identifiable tiles. Go left to right with the mouse as you note down their tile ID:s in a notepad. 
4) Copy the string of tile ID:s from your notepad. 
5) Again in fceux, go to Menu > Debug > Hex Editor
6) select View > ROM file
7) go to Edit > search...
8) Paste the string. If lucky, there's a match in ROM. Now you're very likely to have found the spot in the ROM where that nametable data is stored. You can compare the surrounding hex values with the contents of the nametable viewer to make sure. 
9A) If you're extra lucky, it's a raw nametable without any compression. In this case, you can use fceux hex editor to directly insert .nam data from NEXXT into the rom here. 
 9B) It's also common for title screens to be RLE compressed (or other compression method). Then you'll often find that a row of blank tiles is denoted 00 00 20 or some such, to save space. (00 is the escape character, 2nd 00 the actual character to put, and 20 (in hex) the number of same tiles to draw). 
10). Lastly, save a copy of your edit, and load that copy. You always need to reset or reload before seeing the changes in-game. 

Best of luck!

On another note; be aware that the "put CHR in game" action is Old Code(tm) that the original author of NESST implemented for his specific goals. It copies a whole 8KiB to the ROM, which usually is 4KiB more than you'd want in most cases. Now that i'm aware of this, i'm going to change it so that just the currently active (visible) tileset in NEXXT gets copied to the specified address in future versions.  

thanks