Since you're using QT (
GUI), just launch Bitcoin Core normally and go to console (
Window->Console).
Select the target wallet in the drop-down menu of the console (
if not available, load load it first) and enter the command:
upgradewalletBut in case if you're actually asking how to upgrade it into a 'descriptor wallet':
Select the old wallet in overview's wallet drop-down menu, then use "
File->Migrate Wallet" to convert it into a descriptor wallet.
Wait for it to finish, it may take a while depending on the contents of the wallet file.