How to assign roles to grant permissions to update particular pages?
The Group module allows for creating arbitrary collections of your content and users on your site, and grant access control permissions on those collections.

Group creates groups as entities, making them fully fieldable, extensible and exportable. Every group can have users, roles and permissions attached to it. Groups can also act as a parent of any type of entity. Group provides an extensive API to attach entities to groups.

For way more details about this module, such as how to configure it or available integrations with other modules, refer to my answer to "What are the alternatives for the Organic groups module?".

For this specific question you'd only need 1 Group Type, with 1 Group (of that Group Type) for each set (= collection) of "Specific Pages" (as in your question). BTW, available for either D7 or D8.

Have a look at the Video tutorial "How to use the Group module in Drupal 8 to manage editorial access control to different areas of a web site". Even though it is about D8 (already), the key concepts of it are the same as in the D7 version. Maybe also review the answers to various questions around here tagged with the group tag.

Some more details about this part of your question:


  Some more details about what I'm trying to do: I do not want to give the permission to whole content type (that is not required). I want a user wise permissions like I have a role named ouroffice-user. Then under this I have a 10 users and I want to allocate different permissions to these 10 users like no. 1 have a permission to update the welcome content and no. 2 have a permission to update the about us page all users belong to single role that ouroffice-user.



Be aware that "roles" in the Group module are different from the typical (standard) roles in Drupal. For more details about the various roles and permissions that can be configured, refer to my answer to "What are the various roles supported by the Group module?".
Assume you create a custom Group role like "content manager". And you allow members with this group role to edit nodes of content type ABC.
From those 10 users (in your comment), those that correspond to your "no 1" should become members of some group (say titled Group no 1), in which they receive access to that Group role "content manager". And assume you create a similar group titled Group no 2, for those that correspond to your "no 2" users.
Using the gnode submodule you assign the nodes such as Welcome to the Group no 1 group and About us is assigned to the Group no 2 group. Note that it doesn't matter what the content types are (if they are the same or not), which is part of the beauty of the Group module.


Done ... really! Give it a try to get the idea ... those who try, love it ... after they get it ...