If your motor can be converted to digital shift and throttle -- fly-by-wire -- then there is an RPM Tune button on the digital controls that changes speeds by 50 rpm increments.
"There is never just one thing wrong with a boat"; -- Travis McGee, main character in a book series by John D. McDonald
*************** The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. If you elect to do it yourself, be sure to use caution, common sense, and to observe all safety procedures in the vicinity of gasoline, moving engine parts, high temperature components, heavy items, and electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation or a technique, enlist the services of a factory trained technician.