Q: How can I lengthen my lats?
A: It’s not possible to lengthen any muscle unless you’re not fully grown yet. All that can be done, assuming you’re mature physically, is to develop the length of muscle you already have. You can’t train lower lats and upper lats separately. You train the lat muscle in its entirety. The pulldown, or perhaps a machine pullover if you have a good machine available, may be your best bet until you have the strength to do at least six perfect chinups.
Once you can do at least six perfect reps, you have the option of being able to do conventional chinups, which use a supinated grip, or pullups, which use a pronated grip, in place of the pulldowns or pullovers. Even then, though, the pulldowns or pullovers will still do a good job of developing your lats—provided you work on gradually building up the resistance while maintaining correct exercise technique, for plenty of reps.