standard steps to Restart Seconday server in Logshipping setup
What I usually do with our 15-minute log shipping secondary servers is just wait until the last log is copied over and restored. Then you can reboot the secondary server (assuming that by server you mean a separate physical server). As soon as the secondary comes back up, it will copy over the missing transaction log file and restore them.

We restore our secondary in STANDBY, so some people query them once in a while, just make sure you let the appropriate people know that you're about to reboot the server.