H, I'm just a seller myself, but I worked out how to change your description for single or multiple item.
i\If you receive eBay warning that there is a problem within your listing, you can easily solve it by:
go to your HUB, (my eBay, at the top right corner, don't click but choose SELLING/SOLD......
go to LISTINGS, (under your store and feedback) click ACTIVE.....
click top box or mark all item to be corrected.....
move mouse to EDIT (2.box from left).....
click EDIT ALL LISTINGS.....
mark all item.....
click EDIT FIELDS.....
choose ITEM DESCRPTION.....
choose EDIT LISTINGS IN BULK, ... FIND and REPLACE.....
FIND BOX: type in the word what needed to be changed..... (sample HTTP).....
REPLACE WITH: type in the word/s needed to be replaced... (sample HTTPS).....
click SAVE AND CLOSE.....
Good luck....
