How to change chars over 120?

I have problem with Service Setting (under Change this in theme options in home page). My language use many characters or use over 120. Then i solved this problem by go to wp-content/themes/edu-clean/lib/templates/wp-template/service-mode.php to change 120 to 800.

After theme updated, It will reset 800 back to 120. Please suggestion to solved this problem.

PS. I heard about move some file from Parent to Child them but i don't under stand much. And have some question if i moved or copy some Parent to Child theme, They will have problem in future or not when theme update.