In order to change the author on a blog post, you first need to make sure you have the right Screen Options turned on. On the top right of the screen when editing or adding a blog post, click Screen Options and make sure you have Author toggled on.
Once you've done that, scroll to the bottom of the page and you can select the correct Author from the dropdown.
Can't find the right author name? You may need to add a user! Instructions for doing so can be found here: https://helpme.haleymarketing.com/hc/en-us/articles/200340243-Adding-New-Users-to-your-WordPress-Site