This was also mention in a reply said on another post. Pick a category, such as just men's clothing but not shoes. Click to select all and go to bulk edit. From here you may see some conditions shown in black. These can't be changed because there is a current offer on them. If you think the condition should be changed, make a note of the listing and check back at a later date.
Once in the bulk edit, click on the first listing in the condition column. From here you can go through the conditions of each listing by selecting save and next. If you had entered any comment in the field prior to this change this will help you determine which condition is appropriate. I did this to all my listings when I heard about this from respectable YouTubers. For those that I changed to Excellent, I added the words "no flaws noticed" to let the potential buyer know I've looked at the item listed.
I was able to get through 300 listings in 30 minutes.