Follow the instructions on the download page for the latest stable release. Alternatively, you can also try the preview version.

If for some reason you want to pin an older version of Mono rather than updating to the latest, you can modify the Debian repository to “stable-jessie/snapshots/X.XX.X” instead of “stable-jessie”. For example, “stable-jessie/snapshots/3.10.0” will lock you to that version.

Note: you’ll get a warning similar to this one when running apt update, it is harmless and can be ignored:

These snapshots will cease receiving updates as soon as the next major Mono version gets uploaded - for example, as soon as Mono 3.12 gets uploaded, 3.10 will never receive updates.

On RPM distributions, force the package version in your package manager - all older versions are published in the YUM metadata and should be available.