Step 1: Fix the pronoun.
Suresh is talking about himself, so the pronoun $I$ in the direct speech changes to $he$ in the reported version.
Step 2: Shift the tense.
The reporting verb is "said" (past tense), so the tense inside the reported clause moves one step back. Simple past "did" becomes past perfect "had done".
Step 3: Adjust the time word.
Time words shift too when the reporting happens later than the event. "Yesterday" in direct speech becomes "the day before" in indirect speech, since we no longer know exactly when the reporting takes place relative to "now".
Final Answer:
Putting the pronoun, tense and time-word changes together gives one grammatically consistent sentence.
\[ \boxed{\text{Suresh said that he had done it the day before.}} \]