I don't know of a way to alter the labels on a heatmap. What I would do in this situation is build a custom heatmap-looking plot using a patch object and text objects. Something like this:
% file = "D:\OneDrive\WORK\VALID\value.xlsx";
file = "value.xlsx";
s = [0, 1, 2, 3.6];
vel = [4 4.25 4.5 4.75 5 5.25 5.5 5.75 6 6.25 6.5 6.75 7 7.25];
% NO=0, MIN=0.25, MOD=0.50, MAJ=0.75, COL=1
data_levels = [0, 0.25, 0.5, 0.75, 1];
data_labels = {'NO','MIN','MOD','MAJ','COL'};
tableData = readtable(file, 'ReadVariableNames', false);
[~,idx] = ismember(upper(table2array(tableData)),data_labels);
numericData = data_levels(idx);
% nColors = 256; % Number of colormap colors
nColors = numel(data_levels); % Number of colormap colors
customColormap = [ones(nColors,1), repmat(linspace(0.8, 0, nColors),2,1).'];%RED
edges = [-Inf (data_levels(1:end-1)+data_levels(2:end))/2 Inf];
color_idx = discretize(numericData,edges);
nan_idx = isnan(numericData);
if any(nan_idx,'all')
nColors = nColors+1;
customColormap = [0 0 0; customColormap];
color_idx(nan_idx) = 0;
color_idx = color_idx+1;
cb_labels = [{'NaN'} data_labels];
else
cb_labels = data_labels;
end
labels = cb_labels(color_idx);
x = 0:4;
y = [vel(1)+0.5*(vel(1)-vel(2)), ...
(vel(1:end-1)+vel(2:end))/2, ...
vel(end)+0.5*(vel(end)-vel(end-1))];
xd = repelem(x((1:end-1)+[0;1;1;0;0]),1,numel(y)-1);
yd = repmat(y((1:end-1)+[0;0;1;1;0]),1,numel(x)-1);
figure();
patch('XData',xd,'YData',yd,'FaceColor','flat', ...
'FaceVertexCData',customColormap(color_idx,:))
[xt,yt] = meshgrid(x(1:end-1)+0.5,vel);
text(xt(:),yt(:),labels(:), ...
'FontWeight','bold','HorizontalAlignment','center')
xlim(x([1 end]))
ylim(y([1 end]))
xticks(x(1:end-1)+0.5)
xticklabels(s)
yticks(vel)
set(gca(),'FontWeight','bold')
colormap(customColormap);
cb = colorbar();
cb.YTick = linspace(1/(2*nColors),1-1/(2*nColors),nColors);
cb.YTickLabel = cb_labels;
