In a scenario, you can add 2 steps after the retraining one to automatically deploy the retrained model on API node.
Step 1: Add a build step “Create API service version”.

Step 2: Add a step "Update API deployment".
To link these 2 steps you must use a target variable in step 1 (here target) that you are going to use as the version id in step 2 (here ${target})