Here are some places to start with your revision:
1. Vary your sentence structure. Text is more interesting and more dynamic when it is unpredictable. It becomes quite boring to read a series of sentences that are very similar. Consider:
I walked into the store. The clerk smiled at me. He reached for his notepad. I said hello to him. He said hello back.
As I walked into the store, the clerk smiled at me. He reached for his notepad. When I said hello to him, he said hello back.
2. Circle your verbs. Make sure you have chosen the best action words in every sentence. Did your character speak, or did he declare? Did she climb the stairs, or dash up them? Did he lean against the wall or did he slouch? Small changes make a big difference in the way your scene reads. Vivid verbs help create a more vivid mental picture for your reader. Consider:
picked up vs. grabbed vs. snatched vs. plucked
ate vs. munched vs. chewed vs. gnawed
smiled vs. grinned vs. smirked
3. Watch your adjectives and adverbs. Do this, too, while you are focused on verbs. If your character "walked slowly," maybe he "plodded" instead. It is usually better to use a strong verb (dashed) than a weak verb plus an adverb (ran quickly). Consider:
whispered vs. said softly
sprinted vs. ran hurriedly
smacked vs. hit hard
4. Read out loud. Find a quiet room and read the piece aloud to yourself. If you're comfortable, read it out loud to someone. You will be surprised how different it seems from reading in your head. You will notice parts that are slow, or sentences that don't make sense. You can also catch moments when your word choices seem awkward, or untrue to your narrator's voice.
5. Try everything. Trial and error in revision is a very important process. If a possible change occurs to you, try it. Don't delete your old version, in case you change your mind, but do give the new sentence, paragraph or scene a chance. Who knows--even if it seems strange at first, you may end up loving it! If you don't, you can always go back to how it was before, and you'll feel more confident than ever in your creative choices.