max execution time limit

i bought the pressmart theme from themeforest, then i tried importing a demo theme, which took so long to install, by the time i cheched the imported theme, it didnt come out well, most of the pictures were not showing, i was so annoyed and afraid, because i paid $38 to get it, i then noticed a message i ignored, which was “Current execution time 300 – We recommend setting max execution time to at least 600 for import demo content.” i’ve watched some youtube content, trying to fix it but it’s not working.