Where to Start?
As the comment above asked, how do you know there are usability issues? Why is a redesign necessary? Are there new requirements or are the old ones not being met?
Is there any data/user feedback at all to suggest known problems or desired new features that you can start with? Is there any existing customer feedback mechanism (if not are you creating one)?
Focus on the main problem the app tries to solve & how you know it's working. What is its fundamental purpose (from the user's perspective, not sales or marketing, etc)? Is it doing that thing well or not, and why? Is there another issue customers would rather the app address instead?
Given that you're very familiar with the existing application, you might find it hard to "unlearn" its quirks and difficulties, or to see it objectively as a new user might. You know the expected uses, but perhaps not the unique ways people really use it. The only way to address this is to talk to actual users of the app (or close enough).
Cheapo Usability Testing
If you haven't already, try to find "Don't Make Me Think" and/or "Rocket Surgery Made Easy" by Steve Krug for a quick, cheap, Do-It-Yourself approach to usability testing. The general idea follows:
Find "real" users of the app (or people who fit the target market) and ask them for 15-min to complete 1-2 simple tasks. "Can you use this app to accomplish X? [Done.] Ok, now how about Y?" Let them know your goal is to get honest feedback about the software, not test their ability to use it. This is a 1-on-1 exercise, not a group activity.
Ask them to speak out loud as they explain what they're doing and why. At the very least, take good notes, but ideally ask permission to record video for later reference (get a signed release if necessary).
Just watch how they actually interact with the app as they complete each task. Only offer help if asked or they're hopelessly lost. Try to identify the things that leave them confused, angry, happy, etc. Do your assumptions match what they think/do? Do they try to swipe when you expect them to tap? Does the app's structure lead them where you expect it to, or somewhere else?
Don't spend time asking wide-open questions like "what do you think about this?" or "do you like this color for the interface?" Everybody will give you a different vague answer anyway. Try to elicit clear, actionable feedback you can work with.
When they're done, ask them if things made sense, if the tasks were easy/hard to complete, and what they might specifically change to make it better. Implement what you can & try again later.
Spend a few hours doing this with 5~10 people and you'll have tons more info than you have now! :) Hopefully you'll get some consistent input, which can help guide your design decisions. Repeat the process at least a few more times as you work through the redesign.