How to change order of fields on new item form for SharePoint Online list

Modern SharePoint gives you an easy way to change the order of fields on the user forms (NewForm.aspx, EditForm.aspx, DisplayForm.aspx). You can rearrange the columns within seconds.

1. Navigate to your SharePoint List.
2. Click New to create a new item (you can use any form - New, Edit, Display).
3. From the top right corner, click Edit form.
4. Click Edit columns. Another pane will open with title "Edit columns in the form".

SharePoint new item edit form
5. From here, you can perform various actions
• Show/Hide column on form - check to show, uncheck to hide (applies to all 3 forms - new, edit, display).
• Reorder columns on form - easily drag and drop to move up/down.
• Move columns up and down one by one without drag drop.
• Edit conditional formula - Decide to show/hide field based on value of another field.

SharePoint new item edit columns
6. Once done, click Save. Click cancel to exit without saving.
7. The change will instantly reflect on the screen.

⭐ Reorder SharePoint list columns using traditional classic way

1. Navigate to List.
2. Click List Settings.
3. Click Column ordering.
4. Change the 'Position from top' for required Field name(s) and set the sequence.
5. Click OK.

You can even change the column sequence from the Content Type using column order. But the Modern way is the easiest and quickest.
Avoid using scripts or changing order using tools like SharePoint Designer.
If you have other complex requirement, go for customization using PowerApps or Spfx.