Can I reset my lost iPhone's IMEI?

You can’t “reset the IMEI”.

Without any context, the only thing I can think of that would lead to asking this question would be that you lost your phone, and reported it lost to your cellular carrier. They blacklisted it as a result.

Now you found it and you can’t use it because it’s been blacklisted. Only the carrier who put it on the blacklist in the first place can have it removed.

Note that if you got a replacement phone using insurance, the original device no longer belongs to you. it belongs to the insurance company.

You do not reset an IMEI. That is a number assigned to a device. If you lost the phone, then you need to get a new phone.