The link provided by Isuru does include how to perform update (look for "Change the submodule commit which your repository is tracking")
In short, on the main repository,
- choose the submodules dropdown
- double click the submodule you want to update - you will switch focus to the submodule
- pull (latest) or checkout (specific commit) the required update
- go back to the tab for the main repository
- there will be an uncommitted change (which submodule commit to use) in the main repository
- commit and push the change