Not to worry. It is not difficult to correct this. I don't have MLS in front of me now but it goes something like this. From the main Finances page, go to View Donations (or maybe it's called View/Update Donation). Find the batch that needs to be corrected and click (double-click?) on the name of the batch to view donations. Then click on the donor name for the specific donation to be corrected. This will open a screen just like the one you see when recording a donation on Sunday. Change the things that need to be changed (change the tithing to zero and enter the correct amount for ward missionary). Then click save. MLS will bring up a box asking the reason for the change. Enter something that will make sense to others because it will show that reason on the donor's statement.
When you go to close the window for this batch you'll be prompted to print the batch report again. Go ahead and print it to file with the original paperwork from the day the donation was first recorded. As long as this correction doesn't affect the total dollar amount of the deposit you can make these kinds of changes without a phone call to headquarters. Use the same procedure if you need to change the donor name (for example, a donation was recorded correctly in every way except it was credited to the wrong donor).
My real name is Greg Anderson.