If you're linking to Kit in your YouTube descriptions, it's possible to update them using YouTube Studio!
If you have a duplicated line of text in each of your descriptions that contains a kit link, such as: Check out my gear on Kit! https://kit.com/icecoldgear
Then it's possible to bulk remove that from the description of all YouTube videos, and replace it, in just a few steps!
Remove the old Kit.com link
1. Locate and copy the exact line of text to your clipboard.
2. Open up YouTube Studio > Videos > Select All > Edit (Description)
3. Select the 'Remove' option and paste in your line of Kit text, like so: Check out my Gear on Kit! https://kit.com/icecoldgear. This will only remove text from your videos if the videos contain an exact match of what you typed.
4. Click 'Update Videos' and wait for the process to complete.
Add in your new Kit.co link
1. With all your Videos still selected, select 'Edit (Description)' again.
2. This time, choose 'Insert at end'. Note: It's best to include a linebreak or two here, to ensure the text you're adding is visually separate from other bodies of text. So add an enter or two into the blank field first, then paste in on your original line of Kit text from earlier, replacing .com with .co, like so:
Please keep in mind:
The 'Remove' option on YouTube Studio is case sensitive, so it'll only remove strings of text that are an exact match for what you input. If it's not removing text for you, check your casing!
YouTube Studio doesn't offer an option for adding text to the middle of a description. When using the method outlined above, the new Kit link will end up at the bottom of your description. Check the before/after screenshots above and note how the Kit link moved below the Twitter handle.
If this isn't going to work for your channel, you can do this same job much easier, and with much more control over the end result, by signing up for a TubeBuddy Star account and using their Bulk Find/Replace/Append tool.