I have implemented 2 services A,B in my bundle. I would like to change the code of service A by building a new jar file and do update command but keep the service B running without start it again.
Sounds like you have 2 services in 1 bundle. The unit of deployment is a bundle, so my recommendation is to split the two services into two bundles. Otherwise, undeploying your existing bundle will naturally also tear down Service B.
Alternatively, in case the API/interface resides in a separate bundle, you could deploy a new service-implementation for A in a separate bundle, with a higher priority, and rewire all uses of the service. Which typically is rather confusing, so it's a distant second place recommendation.
Edit: You comment that you are combining services in a bundle to minimize the number of jars, but you want to update the services independently. Specifically for minimizing the number of jars: Are you trying to solve a problem that you indeed had? I'm mainly working with Liferay, which is fully OSGi, and a plain vanilla installation comes with more than 1000 bundles - the runtime handles it just fine. Make sure you're not preemptively optimizing something that doesn't need optimization.
If your components have different maintenance intervals, then deploy them in different bundles. Period. No use working against the system, which has no problem with the number of bundles at all.