The strait-forward way to do this will be like this (well, there should be more sophisticated way...). I hope it will be help you somehow!
% Sample data
[latGrid,lonGrid] = meshgrid(25:45,125:145);
T = table(latGrid(:),lonGrid(:),randi([0,35],size(latGrid(:))),...
'VariableNames',{'lat','lon','temp'});
% Discretize with 5 degree
[latGroupID, latEdge] = discretize(T.lat, 25:5:45);
[lonGroupID, lonEdge] = discretize(T.lon, 125:5:145);
% Add 5 degree mesh ID to the table T
[C,~,ic] = unique([latGroupID,lonGroupID],'rows');
T.meshID = ic;
% Calculate mean temperature in 5 degree mesh
Output = table(latEdge(C(:,1))',lonEdge(C(:,2))',...
splitapply(@mean,T.temp,T.meshID),...
'VariableNames',{'latEdge','lonEdge','aveTemp'});
% Visualize the output
heatmap(Output,'latEdge','lonEdge','ColorVariable','aveTemp');
