The account as you have described it is an asset account not an expense account and you cannot change account types but you can try. First you have to edit that account to make it no longer a sub account. Then you can try changing type but I doubt that will work.
Rename it as something different and add an actual expense account named the same.
To fix the numbers for 2018 all you need is Expense from this 6140 asset to any expense account for the donations that you now add as new