Since we can not install plugins on Jive cloud so we should not upgrade our SSO plugin ?
That is correct that plugins cannot be used in the Jive Cloud environment, so upgrading your custom SSO plugin would not be recommended.
What will be recommended way to upgrade SSO plugin ? Is it using Jive app or something ?
For Jive Cloud instances SAML SSO is supported out of the box. For more information on SSO in cloud you might also find the Single Sign-On section of the Cloud Documentation helpful.
Additionally, if you have specific questions related to migrating or converting your custom plugins so that they are compatible with a Jive Cloud environment I would recommend reaching out to the Project Manager who is assisting in your migration/transition from a Jive Custom to Jive Cloud build. If you are unsure who the PM assigned to this project is please file a case in your customer support group and the next available support engineer can assist in confirming this information for you.