If my ablution, which I performed for the Fajr prayer, is not nullified, and the Dhuhr prayer call is made, how can I renew my ablution without performing it again?

It is permissible for a Muslim to pray more than one obligatory prayer with a single ablution, so long as his ablution is not invalidated. However, it is desirable to renew the ablution for each prayer to follow the Sunnah and attain reward and recompense. The Prophet, peace and blessings be upon him, used to perform ablution for every prayer, and on the day of the Conquest [of Mecca], he prayed the prayers with a single ablution, demonstrating its permissibility.