You need to deal with each voice (the triplets and the non-triplets) separately, and don’t worry if some of the intermediate steps look at mess when you have only done half the changes.
Change the time signature to 9/8. To get rid of the triplets, select insert mode (press I) then select just the "3"s (not the notes in the triplets!) and press delete.
Now you need to make the non-triplet notes longer. If you had some quarter notes, you can select them in insert mode and press . to change them into dotted quarters.
That doesn’t work for your long notes because you can’t represent a full bar of 9/8 time with a single note duration, so the best way is probably select some notes which all have the same duration (e.g. the two-bar chords in your picture) and use shift-alt-right arrow to lengthen them.
Just work your way from the start stretching the notes to the correct length, and don’t worry about what the unprocessed part of the score looks like. Unlike other notation software, Dorico “knows” that a set of tied notes is really one long note, and it can deal with triplets that get split over a bar line etc while you are working through the score.