Welcome and here are some step by step instructions - the "easiest" I can come up with...
Summary: you need to completely wipe out the BlackBerry, and reload the operating system and programs on the BlackBerry.
Step 1: Search for and download a program called "JL_CMNDR" on this forum. Use it to COMPLETELY wipe out the 7100i; the 7100i will show a "507" code on it now - that means it has no operating system.
Step 2: Download the latest 7100i software from the Nextel web site. The URL is https://www.blackberry.com/Downloads...B9D5EE16489088
Step 3: Execute the file you just downloaded in step 2:
Step 4: Restart Desktop Manager with the now empty 7100i connected. It should reload the OS. This could take up to 20 minutes or more to complete.
That should do it - good luck!