Hi, welcome to the forum!
The specifics can vary. I assume you are using the editor on osm.org (iD) and you are on a PC.
After selecting the node, you can:
- Right-click on it and select the trash can in the context menu.
- Hit the “Delete” button on your keyboard.
- Use the key combination Ctrl+Backspace.
If you have questions or need further guidance, feel free to ask. It may also be useful to know the node (via link) or what kind it is. Sometimes, deletion is not the best way forward.