public class DemoBlock extends BlockWithEntity {

    [...]

    @Override
    public ActionResult onUse(BlockState state, World world, BlockPos pos, PlayerEntity player, BlockHitResult hit) {
        if (!world.isClient){
            BlockEntity blockEntity = world.getBlockEntity(pos);
            if (blockEntity instanceof DemoBlockEntity demoBlockEntity) {
                demoBlockEntity.number++;
                player.sendMessage(Text.literal("Number is " + demoBlockEntity.number));
                demoBlockEntity.markDirty();
            }
        }
        
        return ActionResult.SUCCESS;
    }
}