Not much you can do - you can't cut into old wood, it'll kill the plant, all you can do is trim over lightly a couple of times of year after main flush of flowering, taking a little leaf at the same time. Lavenders usually need replacing after 5-8 years because they become leggy, woody and unattractive, especially if they were never pruned or trimmed regularly.

Easy enough to take cuttings about now to replace the old bush.

I have several self seeded lavenders from my rather old bush. One of them chose a big pot with encomis in it. It is now huge , as the pot is large, they are quite happy together, both blooming well. There are a couple in an old sink too. Have a look before you hoe or tidy up. Just in case.