There was a table manager plugin in earlier versions I can't see anymore.
Is there a way to achieve this in QGIS 3.6?
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See my answer here: QGIS changing the attribute input fields order
Open the attribute table. Right click the column headings in the attribute table. Select "Organize columns" in drop down menu. Then, just drag the heading to the position you want it to be in (See image of the "organize table columns" box below). Click OK. Headings should have rearranged the way you want it to be arranged in the attribute table.