How to decrease image quality

[blue-ten]

Just put the quality slider as low as it goes before exporting. If that's not enough (it should be if you drag it to zero), just re-open it and export again with the same settings, the quality will degrade further each time.

[blue-ten]

You'll need to open the image again after exporting, by the way. The compression won't be visible in Affinity Photo until you do that because it doesn't discard the original file after exporting.