It can be done in pieces. retime applies the same method to all timetable variables. You'll need to create new timetables (only containing the variables you need for each method) and call retime for each method you want to use (first value, max, min, last value, sum) . At the end, combine the smaller time tables together. See this example from the documentation.
If I were to do this with the table you show above, I'd do something like this.
% Create a timetable for each method only containing the variables that method is applied to
M15_open = M15(:,"Open");
M15_high = M15(:,"High");
M15_low = M15(:,"Low");
M15_close = M15(:,"Close");
M15_vol = M15(:,"Volume");
% Retime each timetable
H1_open = retime(M15_open,'hourly','firstvalue');
H1_high = retime(M15_high,'hourly','max');
H1_low = retime(M15_low,'hourly','min');
H1_close = retime(M15_close,'hourly','lastvalue');
H1_vol = retime(M15_vol,'hourly','sum');
% Combine the individual timetables back into a single time table
H1 = [H1_open H1_high H1_low H1_close H1_vol]
