1. First, open PPT, select the slide with subtitles to be changed, click the text box button in the Insert tab, and then draw a text box on the slide.

2. Then move the cursor to the text box and enter the new subtitle content.

3. Secondly, if you need to adjust the position and size of subtitles, you can select the text box, and then drag or adjust the border size with the mouse.

Finally, you need to change the font, color and other styles of subtitles. You can set them in the Start tab and preview the slides to ensure that the subtitles have been successfully replaced.