First of all, the title is not clear.
I found the answer which is called resume training. (https://www.mathworks.com/help/deeplearning/ug/resume-training-from-a-checkpoint-network.html)
But my condition is different from the offcical website example. I used function trainFasterRCNNObjectDetector and the type of my network is DAGNetwork. As a result, the solution for my situation is shown below
load('rcnn.mat'); % Load previously trained network.
net = layerGraph(rcnn.Network); % Transform DAGNetwork to make it excutable for trainFasterRCNNObjectDetector.
rcnn = trainFasterRCNNObjectDetector(trainingImages, layerGraph(net), options);
save('rcnn.mat', 'rcnn'); % Save trained network.