I assume this is for a Rye bread (Dill Rye is one of my favorites!). Seed and weed have a slightly different flavor and the seed adds more of a textural component. But otherwise, it won't hurt anything to swap them.
You might also try to substitute caraway or fennel seeds.
I don't know the quantity of dill seed the recipe calls for, but it seems to me that 1 tsp. of dill weed will have much less flavor than 1 tsp. of dill seed, so you may need to adjust the amount.