Hello, I am new to K2 and Joomla! . My problem is when I update K2, all my template overrides are gone! So how to keep the template overrides even after update??? Please help :(

If you put them in the proper place, updating k2 should not overrite them.
they should be in
YourSiteRoot/templates/YourTemplateName/html/com_k2/templates/YourTemplateDirectory/{files}