Explain the changes in spelling from Old English to modern English.
I'm guessing you're asking for the cause of these changes? At any rate, up until the Dictionary of Samuel Johnson in the 18th Century, everyone in Europe just wrote and spelled phonetically. There really was no standard for spelling or writing anything, which is why you will find several different spellings of words and names...even Shakepeare's signature is spelled several different ways.
Part of the reason for all the confusion is the fact that English is a modge-podge of so many different languages and language bases...Latin, German, French, Celtic, Welsh, just to name a few. Another reason there are differences in pronunciation and spelling is due to an event called the Great Vowel Shift which happened during the 15th to 18th Centuries which helps explain why we now have so many silent "e's" and "k's" in our language, making these words a little more difficult to spell.
For example, in Chaucer's time, the word "knife" was pronounced phonetically as (hard "k") "ka-ni-fa". Now, the "k" and the "e" are silent, making the pronunciation "nief".
Hope this helps a little! I've included some interesting links which might shed a little more light on the subject for you. Good Luck!