Hi Hemant
Thank you for posting your query in Symantec community.
I would be glad to answer your query.
Actually there is not any supported method to downgrade the SEPM, you will have to build it from scratch.
Follow these steps
1) Uninstall existing SEPM with database.
2) Install SEPM 12.1 RU4 & restore the database, support can provide your SEPM 12.1 RU4 Setup, or just ping me the case number I will make sure to send you download links
3) Restore 12.1 RU4 database
4) SEPM and SEP clients communication should work if there isn't any change in communication file or else will have to push out communication update package.