You can change a header or footer placeholder by resizing it or moving it.
Change one slide or many slides?
When you resize or move a placeholder on a slide, the change occurs on that slide only. To resize or move placeholders on multiple slides, perform the procedures below on a Master View (either Slide Master, Handout Master, or Notes Master) on the View tab of the ribbon.
Changes you make to a master apply to all new slides added to a presentation, and changes make to a slide layout apply to all instances of that slide layout that you add to your presentation.
Resize a footer placeholder
Select the placeholder that you want to change, point to one of its sizing handles, and when the pointer becomes a two-headed arrow, drag the handle.
Move a placeholder
Select the placeholder that you want to move, point to a border, and when the pointer becomes a four-headed arrow, drag the footer placeholder to a new position.
To format handouts, see Edit page numbering, footers, and headers for Handouts.
To format notes pages, see Edit page numbering, footers, and headers for Notes pages.
To add slide numbers, page numbers, or the date or time to your slides, see Insert or change the slide numbers, date, or footer in PowerPoint slides.
To change the font, size, case, color, or spacing for text within a placeholder, select the text, select the Home tab, and then, in the Font group, choose the formatting options that you want.
You can learn more about slide masters and slide layouts in these articles: