Okay, there are many ways in which you can edit a tileset. I will explain three.
Make sure you have everything ready. You will need AdvanceMap, the ROM of your choice, and a paint program.
Open the ROM in AdvanceMap, go to the map that has the tileset you want to edit, and go to the block editor.
While in the block editor, choose the pallette of the building you want to insert.
While still in the block editor, go to Picture (It is in the upper-left corner). While there, go to Save Tileset 1 or 2 (Depending on which one you want to edit).
Open the tileset you just saved. Insert the building, and use the eyedropper tool to make sure the tile has the correct colors. It is very importanat thet the colors are correct. Now, save the tileset.
In the AdvanceMap block editor, go to Picture. Go to Load Tileset 1/2 (Depending on which on which one you saved and edited). The new tileset should appear. If not, go to Pictures, then to Reload Blocks.
Edit the blocks. Also, if needed, fix the map footer. There! Your done!
.: 2 - TileLayer Pro :.
This is the safest, but hardest way to edit tiles.
Make sure you have everything ready. You will need unLZ-GBA, the ROM of your choice, guide tiles, and TileLayer Pro.
Open the ROM with unLZ. Open the tileset. (To do this, you may need to run a Deep Scan)
Go to File, then go to Raw Dump. When it says Save As, click on the down arrow where it says RAW, and select All files. Save it as X.gba, where X is a different name than the ROM (It is important that you save it as a .gba file. That way, TLP recognizes it.)
Open the new raw dump (X.gba). Also, open Paint with the tile you want to edit, and find a guide tile (Or tiles).
Edit the tiles, then save as X.raw. Open unLZ, then go to the tileset. Go to Load Raw and choose X.raw. When the check boxes appear, check all of them except Abort If Data Is Bigger. There! Your done!
.: 3 - unLZ and Paint :.
Make sure you have everything ready. You will need unLZ-GBA, the ROM of your choice, guide tiles, and Paint.
Find the correct pallette (If you cannot find the correct pallette of the tile you want to replace, then find a 16-color pallette. You will need to use a guide tile for this). Click on Save As...
Open the tileset in Paint. Then insert the tile, making sure it is properly alligned with the other tiles. Then use the eydropper tool to make sure it has the right colors. Then save the tileset.
In unLZ, go to the tileset. Make sure the pallette is the same as when you saved the tileset. Then click on import. There! Your done!
.: Notes :.
~Due to differences in ROM dumps, I just generalized instead of giving specific Goto numbers and Pallette numbers.
~A guide tile is a tile that shows what the colors of TLP are in-game. For example, if replacing a mountain tile, you would use the pre-existing mountain tile to guide what colors you would need to insert the new tile as.