Hi ​@Rlks816, if it's a gps activity (run, bike ride etc), you can open the activity in the app, then tap the three dots in the top right corner, and then tap Crop Activity. You'll be able to adjust the length (both distance & duration) by moving the start and/or finish slider. 
For non-gps records, it's not possible as far as I know. 

Thanks for your reply. It’s a non gps activity - weight training class. 

That can't be changed I’m afraid. However, there's a pending feature request that would bring (also) this functionality, you might want to vote for it: