The reason both notes are highlighted is that in Dorico, tied notes are really just one note that’s notated as a tie depending on its duration, position in the bar (Dorico automatically splits notes into ties across the half-bar, for example) and position relative to barlines (similarly Dorico splits notes into ties when they extend across bars). So instead of thinking of “changing the quarter note to a half note”, in Dorico you can instead think “I want to lengthen that note by a quarter note”, and the key command Dan described does just that. If necessary, Dorico will renotate the note as appropriate for its new duration.
You can control how notes and ties are split on the Note Grouping and Beam Grouping pages in Notation Options (press Ctrl/Cmd-Shift-N), which sets your defaults on a per-flow basis. You can also use Force Duration locally to specify exactly what note durations you want displayed if necessary.
(If you ever want to put something in the middle of a tie chain, like a dynamic, show the note input caret by pressing Shift-N and press Space/the arrow keys advance the caret to where you want the e.g. dynamic to go, then input it with the caret still showing.)