If team selects works only for first few days then?

I'd suggest you review the guide once again to get a better understanding why there's a single sprint planning with the development team forecasting what it can achieve in that particular sprint.

Now, if all work is complete, per the team's definition of done, before sprint ends, the scrum team can obviously see what to do with the remaining time - fix some technical debt, grab a few tickets from the backlog, address outstanding bugs. But this is a totally different story that the one you mention

@Eugene But in Scrum Guide only it's mentioned that team can select work only for first few days.

But this is a totally different story that the one you mention

Right. I want to know the scenario in the case I have mentioned in first post. 

@Vishal:

The Scrum Guide states: "However, enough work is planned during Sprint Planning for the Development Team to forecast what it believes it can do in the upcoming Sprint. Work planned for the first days of the Sprint by the Development Team is decomposed by the end of this meeting, often to units of one day or less."

By "Work", the Scrum Guide does not refer to the Backlog Items  on top of the Product Backlog, which the Dev Team selects for their sprint. It refers to the activity of breaking down said Backlog Items into actionable steps. Scrum refers to THIS, specifically, as Work. In some circles, these are called "tasks". 

The "Work" portion of the Sprint Backlog may be changed during the sprint, at the discretion of the Development Team, who own the Sprint Backlog. "Work" related to a specific Backlog Item may be added or removed as the Dev Team's understanding of how to make a specific Backlog Item "Done".

Therefore, it is OK for the team to do this detailed breakdown at the Sprint Planning just for the Backlog Items they intend to work on first; but then they will need to perform this activity for the rest of the Sprint Backlog during the sprint, as this becomes necessary. 

I believe you are misinterpreting the Scrum Guide.

At the Sprint Planning, one of the things that the Development Team does is selecting the Product Backlog Items that they believe that they develop over the course of the Sprint. These Product Backlog Items become part of the Sprint Backlog. However, to promote transparency, the Sprint Backlog is more than just the Product Backlog Items, but also work items that have been decomposed from Product Backlog Items. At Sprint Planning, the Development Team decomposes enough Product Backlog Items for a few days, and continues to decompose Product Backlog Items that were placed into the Sprint Backlog throughout the Sprint as work progresses.

Only the Development Team can change the Sprint Backlog. If the Development Team is simply decomposing the work, the team should be able to do that on their own. However, if the Development Team determines that they cannot complete the work in the Sprint Backlog, they should work with the Product Owner to prioritize the work to deliver the most value at the end of the Sprint. If a Development Team can bring on more work into the Sprint Backlog, they should also work with the Product Owner. The Scrum Master can help facilitate all of these - helping the team to effectively decompose the work and ensure transparency or working with the team and Product Owner to prioritize and update the work being done.

I believe you are misinterpreting the Scrum Guide.

Hi Thomas, much clear now... The work is chosen for entire sprint but only decomposition of work happens for first few days.. Got it.

Thank you,.

That doesn't mean that work is then fixed.  The work chosen is a forecast.