Purchased some credits, but dont know how to modify team name, colors, stadiums. Anyone direct me please.

If customisation is possible in your league, go to team page. It's just below owner, future draft picks, etc.

Can only be done in the offseason as well ...

If you are in a user-generated league, it might or might not work, depending on whether the admin does customizations.
In a custom league, it will always work.
In a regular league, it will never work.
Customizations are only allowed before the pre-season. At that time, a "Customize" button will appear on your Team Home Page, underneath future draft picks. Then you will get a menu of how many credits to spend to rename your players, move your team, change your colors, etc., etc.