If you need to monetize a large amount of videos at once, you can change the settings in bulk rather than editing each video individually.  To start, head to your channel's Video Manager page.

At the top of the page will be a checkbox next to the 'Actions' button.  Check this box to select all videos on the first page. 

After checking this box, an option will appear on a yellow bar to 'Select all your videos.' Click this link in order to select all uploaded videos on your channel including those on other pages.

Selecting all of your videos will automatically open an expanded 'Actions' menu. Note that if you have only selected some of your videos, you will need to hit 'Actions' and then select 'More Actions..." to see the full list of options.

On the expanded menu, first select 'Usage Policy' to make sure monetization is enabled for all your videos:

On this page, make sure the dropdown field for Usage Policy it set to 'Monetize in all countries' and then press the blue Submit button.

After a while, the page will display a message telling you when all of your videos have been processed.

At this point, you can change your Ad Formats for your videos to manage what ad types will be displayed on your videos. Repeat the first few steps again to select your videos and access the expanded 'Actions' menu.  This time, select the 'Ad Formats' option.

This will bring up a page to allow you to select what ad formats you wish to use on your channel's videos:

We recommend checking the boxes for every available ad type in order to maximize your results.  However, you are free to select whichever options you prefer for your channel.

When you are finished, hit the blue 'Submit' button to save the changes. The page will display a message telling you when all of your videos have been processed with these changes.