, if you are already on 15.7(3)M2 or M5, review the release notes. The M8 update fixes a handful of medium-severity bugs but does not add new features.
The "M" in the version name indicates a release, optimized for reliability rather than introducing experimental features. i--- Download C2900-universalk9-mz.spa.157-3.m8.bin --
show flash:
You likely need to run license boot module c2900 technology-package securityk9 and reload. , if you are already on 15
If you have a PAK file from Cisco: