Redefine functions

Hello. How can I redefine functions in the next files in prespa theme:

https://ww.wp.xz.cn/support/theme/prespa/
 /inc/template-tags.php
/template-parts/content-excerpt.php
/inc/custom-header.php – prespa_header_image_css()
/inc/template-functions.php – prespa_display_related_posts()

Each theme update breaks my site because of changing these files.

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