@Sangeetha
If you're uploading your products with a direct data feed from your website, each time it refreshes it will override the changes you have made on Facebook - including the description.
You would have to make the changes in Squarespace (but that might not fix the messed up text) or add a supplementary feed to override certain attributes, like the description. This feed is generally just a csv or sheets/excel file with a product identifier (e.g. SKU) for each row and a field you want to change in each subsequent column.