SEPM reinstall help

We have a SEPM , but manager is not updating the signature. and we were running 12.1.671, as told. we believe it may help if we install 12.1 ru2.

However, we notice that our DB maybe completely useless. if we want to remove the historical sem5.db completely, what's the exact steps?

If we reinstall 12.1 ru2 into the very same SEPM. Will the managed clients come up in the 12.1 ru2 after reinstall?