How do i change the font on something I’ve already typed on Pages?

Select the text you wish to change.

Click the format brush to open the Format Inspector in the right side bar.

Choose Text.

Click Style.

Click the Font menu bar.

Select the new font from the list of available fonts.

Click the Format brush to close the Inspector.

Regards,

Barry

As your profile shows an iPad Pro, here is an exerpt from the Pages for iPadOS User Guide that may help. First, select the typed text, and then change its font attributes:

Change the look of text in Pages on iPad - Apple Support