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This question already has an answer here:

I'm wondering how can I set a background color to white instead of alpha?

I've already tried more things.

marked as duplicate by Cai Jun 30 '17 at 18:00

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    Can you elaborate on what you mean by "more things"? At this time, the question is written it may be likely closed due to no effort and seems to be seeking a tutorial. – Mᴏɴᴋᴇʏ Jun 26 '17 at 21:54
2

Two possible ways:

Bucket-fill:

  • Remove the selection
  • Set color to white
  • Activate bucket-fill
  • Set bucket-fill to "Behind" mode
  • Bucket-fill (due to "Behind" opaque pixels will be left untouched)

Layer merge:

  • A new layer, fill with required color
  • Drag to below the target layer in Layers list (or use Layer>Stack menus)
  • Activate the target layer and Layer>Merge down

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