If your tent's seam tape is flaking and you are unable to pull off the seam tape by hand, you can use an old toothbrush to brush off the flakes off. For stubborn areas you can use a solution of rubbing alcohol, dish soap, and water in a small basin or bucket. Wear gloves and soak a rag or washcloth in this solution and use it to wipe away the degraded seam tape. If a few flakes remain here and there, that's OK. You can apply the sealant over these areas. The idea is to remove as much of the flakes as possible, but it's not absolutely necessary to remove 100% of it.
We do not currently offer resealing or reapplying waterproofing to tents. We recommend Nikwax and GearAid products to complete the repairs at home.
Another option is to replace the rainfly or canopy with a new one. We do carry replacements instead of making repairs at home or if the rainfly still leaks more than you would expect. Check out this article for more info on tent repairs and replacement parts: Tent Repairs and Replacement Parts.
Resources:
Video: How to Seam Seal Your Tent
Article: How to Wash a Tent (*Includes instructions on how to reapply a waterproof coating spray)
Article: Seam Taping vs Seam Sealing
Article: Tent Care & Maintenance: How to Make Your Tent Last Longer
Article: 3 Ways to Field Repair a Rip in Your Tent's Rainfly