A plant is only as healthy as its root system. When roots become entangled and squashed, the plant can no longer access water or nutrients. Further, once the root system has filled a pot, you'll find less and less soil. Nothing to keep the plant alive!

With such a small pot, the only answer is to repot to a larger pot (at least twice that size); OR to divide the plant and repot some of it in the same pot. Obviously there'll be enough roots to create several new plants.

Just tip out the pot, plant soil and all. Grab a handful (more, or less) of what looks like healthy plant plus roots (or, in the case of mint, just roots or a runner), tidy it up a bit by trimming extra long runners and gangly stems, and plant it. Make sure if using the same pot that you give it a thorough clean, using soapy water, and rinsing in clean water with a dash of bleach. This treatment kills any residual fungal diseases that may be lurking, also any bugs.

It's as easy as that.

I would just divide it and put into 2 pots - if you don't want 2 - take the healthiest of the two and give one away.

Thank you so much. I don't know why I let the mint roots intimidate me. Your responses very very helpful and supportive. :-)