The Posuk says:
וַיִּקְרָא אֹתָהּ שִׁבְעָה עַל כֵּן שֵׁם הָעִיר בְּאֵר שֶׁבַע עַד הַיּוֹם הַזֶּה
And he named it Shibah; therefore, the city is named Beer sheba until this very day
The problem is earlier in Parshas Vayera (בראשית כ"א ל"ג) it says:
וַיִּטַּע אֶשֶׁל בִּבְאֵר שָׁבַע
Translated as:And he planted an eishel in Beer-Sheba
so it had the name earlier so why does the torah say "therefore, the city is named Beer sheba" if it already had that Name?