No worries mate :P
Basically, if you have new tiles inserted into the tileset (2) then you're going to have to change the blocks (1) by using the block editor (3). You can expand the number of blocks you can have in your second tileset by going to 'Blocks > Change Amount' and then entering a larger number (but be aware that a larger blockset will take up more data) if you don't want to overwrite any existing blocks.
Anyway, to transfer the tiles from the tileset to the blockset, select the block that you want to overwrite and the rectangle in the block editor should change. If you click on an 8x8 tile in the tileset and then click on the block editor you should see that it instantly changes to include that tile where you clicked it. Click save in the block editor and the block in your blockset should change.
Sorry for giving you a very short explanation, but hopefully it helps :P