Thanks Akash, it worked well. My only problem now is that I have photos marked as taken 2015 01 31 and I don't know what to put for the date witch is 1 month later.
Where does it says Taken on 2015 01 31?
Capture date in metadata Panel cannot be changed
You need to change the EXIF metadata using a different Application/Tool.
Use one you'd like Change Exif metadata
Then have to read the metadata in Lightroom again from Metadata > Read Metatadata From File.
Huh. Changing capture date is trivial using the Metadata->Edit Capture Time thing. Don't know why you say it isn't possible. You don't need any external tools to do it. You can change the date in Lightroom itself without issue using the menu item. If you use the default option (adjust to a specified date and time) all the images shift to the new time and date but retain their relative time difference to the topmost selected image. The interface is not clear about that but that is what it does.