You should be presented with a sub-window that lists your various donation dates and charities, together with amounts. Included on each line should be "Add" "Edit" "Delete". I've used Edit to add missed entries or (in one instance) change the charity name because the original charity didn't show up to pick up the donation so we gave it to a different one.
I've been able to do this as well in 2018 (for 2017 donations).
If you're not getting that listing or if the buttons don't work, you could try a different browser. I've used both Chrome and Edge (both on Windows 10) but your experience might differ.