how to change word "This entry was posted in blog on" newest version 3.9.1

First, confirm that whatever theme you are using supports translation.

Assuming your theme does, you need to tell WordPress which language you are using, as explained here: https://codex.www.ads-software.com/Installing_WordPress_in_Your_Language

Once you have setup WordPress to use your language of choice, you will need to create a translation file. Providing your theme comes with a .po file containing the English strings used in the theme, then the process is pretty straight forward. For more information on creating a translation file, please review: https://codex.www.ads-software.com/Translating_WordPress

thanks