First, update to the latest minor version of the 9.8.x branch and then jump from the closest esr to esr (extended support release), and don’t skip one as stated in the docs:
Mattermost will aim to have non-locking, backwards-compatible migrations in general. This backwards compatibility guarantee extends to only the last ESR version. For example, if there are three ESR versions ESR1, ESR2, and ESR3, upgrading from ESR1 to ESR2, and then ESR2 to ESR3 will ensure backwards compatibility, but not from ESR1 to ESR3 directly.
In the case of delayed upgrades, we recommend upgrading to the closest ESR version first, and from there to the next ESR. Do not attempt to directly upgrade to the latest version as it might break backwards compatibility of the older nodes in the cluster during the upgrade.
=> Prepare to upgrade Mattermost - Mattermost documentation
Good luck