If for some reason, a previous version of OSLC Connect for Jira needs to be installed, you only need to install it using the UPM (Universal Plugin Manager).
Before downgrading, please consider reporting any inconvenience or issue to a support representative; this to check whether an upgrade to an improved version is possible in the short term, or to confirm otherwise that the downgrade is the path to follow.
Once downgraded, and if IBM Engineering Lifecycle Management global configurations or IBM Jazz Reporting Service / IBM Engineering Lifecycle Optimization - Engineering Insights (ENI) are being used by any Jira project administrator, advise the IBM Link Index Provider (LDX) and IBM Lifecycle Query Engine (LQE) administrators to verify the Jira datasources and to re-index in the case an error status was reported after the downgrade. This is a very important task in order to keep synchronizing those applications with Jira. On Reindexing an LDX/LQE datasource topic, LDX/LQE administrators can find a guide to do it.