Hi Aakash,
You can create your own side menu with the name 'SideMenuCustom' as webblock and then find the usage of the default sidemenu, Now open the screen where you want to change this click on widget tree and open navigation placeholder remove the default 'sidemenu' webblock and replace by your navigation webblock 'SideMenuCustom' .
I have changed the default menu with menu2 in the attached image.
Thanks