Always spawn in the same place in the Nether (Minecraft)?

Although I have already dismantled the portal and placed it a little further away, still in the same place in the Nether as before.

Can someone tell me how to spawn somewhere else?

Dismantle the portal in the Nether and place a new one where you want to spawn.

In case you are interested, portals work like this:

If you go through a portal, the game calculates where you would get out in the other dimension (by taking the coordinates times 8 (Nether → Overworld) or dividing them by 8 (Overworld → Nether). Then it looks to see if in one In a certain radius (Nether: 128 blocks, Overworld: 1024 blocks) a portal already exists, and if so, then you will simply be teleported to it instead of a new one being created.