Select the person for whom you want to set their mother
Click <Change or Delete> button in sidebar
Click <Change parents> button in sidebar
Click <Choose new parents from tree> button in sidebar
Click in the white field below "Choose the new mother" and wait for a list of names to appear
You can scroll up/down to the name (they're sorted by lastname then firstname) and then click on it
Click <Set new parents> button in the sidebar