Siman 220
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א הָרוֹאֶה חֲלוֹם וְנַפְשׁוֹ עֲגוּמָה, יֵיטִיבֶנּוּ בְּאַפֵּי תְּלָתָא דְּרַחֲמֵי (פי' שֶׁאוֹהֲבִים אוֹתוֹ) לֵיהּ, וְלֵימָא בְּאַפַּיְהוּ: חֶלְמָא טָבָא חַזַּאי, וְלֵימְרוּ אִינְהוּ: טָבָא הוּא וְטָבָא לִיהֲוֵי וְכו'.
1 Rectifying a dream and fasting about it. Contains two paragraphs. When one has a dream and is anxious about it, one can rectify the dream in front of three people who care about him. He can say to them: "I had a good dream" and they can say to him "it is a good dream and it will be well."
ב יָפָה תַּעֲנִית לְבַטֵּל חֲלוֹם רַע, כְּאֵשׁ לִנְעֹרֶת. הַגָּה: וְדַוְקָא בּוֹ בַּיּוֹם, וַאֲפִלּוּ בְּשַׁבָּת, וְעַיֵּן לְקַמָּן סִימָן רפ"ח.
2 It is good to fast to nullify a bad dream. Like fire on wood chips. Rema: Specifically, this applies to the same day and even on Shabbat.