I couldn*t find a tool similar to the Remove rests plugin in Finale. Is this hidden somewhere or planned for a future update? By the way, in Finale this functions omits tuplets, which makes it less useful, so I’m looking for something better in Dorico 
I think the topic of “hiding rests” is the most discussed in this forum. There might be a thread opened every two days about that… Please, use the search engine 
In Dorico, rests are removed using the properties of the voice which has the rests, ending the voice before the rests and starting the voice after those rests.
I did search, I can assure you… My question was not about hiding rests - I was looking for a function that removes/replaces rests by making the previous notes longer. By the way, I was referring to Finale, I did mean the Remove rests plugin in Sibelius.
You have a very nice function in Dorico : alt-shift and left-right arrows. Alt-shift-right arrow increases the length of the selected notes by the “quantize” rythmic value (left down corner), alt-shift-left arrow decreases this length by the same amount. Maybe the trick with the right arrow could solve your problem !
Of course all this works in Write mode (cmd/ctrl-2)
If you want to change the duration of existing notes, then, well, change the duration of those notes. The rests will be taken care of automatically, always.
Select the notes, then choose the desired duration in the panel on the left. Alternatively, as Marc mentioned, Shift+Alt+left/right will change the duration by the amount set in the rhythmic grid. Try with insert mode enabled/disabled, depending on what you’re trying to do musically.