Tulsie needs to be tipped to keep it from going to
seed, which is the signal for the plant to die back.
Basil is not perennial but it does reseed itself, just let the flowered plant(s) make their seed and drop them (or gather the seeds if you want to spread more around the garden).
Since those you have are in flower, go ahead and pluck and use the leaves, but the plant is headed towards the end of its life now.