Installing Ubuntu after Windows 10 [duplicate]

How can I install Ubuntu on Windows 10 as dual boot? I have a DVD with Ubuntu and when I tried to install Ubuntu I had a warning that the hard disk will be erased. How can I install Ubuntu and keep my Windows 10? Thanks

Since you probably don't want to erase your Windows.

First step would be not to follow recommended steps during Ubuntu installation, and choose custom one.

I would recommend first to create partition in windows boot for linux, don't place it in the middle and non less than 50GB (despite the fact that Linux is light weight, it never cleans it's garbage, so with 50GB you will not have to clean it for next year or 2). Once you have partition for linux, at this point you can start instaling Ubuntu.

During custome instalation set SWAP area on linux partition to your RAM size, everething else to Ubuntu ROOT and linux formating (should have number 4 at the end), and under root I mean '/'.

Before proceeding it will show you what change will be applied.

Something to note: Ubuntu will take you BIOS time, thus on Windows you'll be 1 hours late and if eventually you would want to roll back to solo windows boot it's not straightforward.