If something went wrong with your Stryd (for example if your weight setting was incorrect on Stryd), you can adjust the recorded power after your run. You can do this if you have a good understanding of what percentage your power is off. NOTE THAT THIS DOES NOT WORK WITH GARMIN RECORDED DATA!
This is not possible within PowerCenter, but you can use an external editor to manipulate your run data and cut the part you want to remove or split your run into multiple sections. The recommended free tool is https://www.fitfiletools.com. Note that you can also combine files and other manipulations with FIT files (not all possibilities are covered here, contact us for assistance if needed).
Export your FIT file in PowerCenter to your hard disk.
If step one was successful, delete the activity from PowerCenter.
Open a tab in your browser, and open https://www.fitfiletools.com
Follow the instructions on the web page (use the power adjuster tool). Save the file from fitfiletools to your hard disk. If you want to split your run data, just repeat this step so that you end up with two separate files.
In PowerCenter, go to the Calendar Page, press the top right icon, and press Upload Run. You will be able to upload FIT files. Upload the files saved in step 4. Your modified run data will now appear in PowerCenter.