How to repair warped sliding door frame?
To me, “a warped frame” means twisted. In order to “fix” a twisted frame, you’ll need to 1) remove the track (threshold), 2) clean the track and where it’s placed, 3) straighten the track, and 4) re-secure the track to its correct position. 

Straightening the track could be the most difficult. I’d try heating the track so you can twist it back to it’s original shape. I think a tool like an electricians conduit bender will work. (We use it to heat and bend Trex Decking too.) It looks like this and you could put the “warped” end of the frame into it...

https://jet.com/product/Greenlee-PVC-HeaterBender/1519c3795e4b4943bccec436d81b0447?jcmp=pla:ggl:nj_roc_gen_appliances_a1:appliances_heating_cooling_air_quality_space_heaters_a1:na:PLA_1062463102_53743929562_pla-306111780120_m:na:na:na:2PLA15&amp;pid=kenshoo_int&amp;c=1062463102&amp;is_retargeting=true&amp;clickid=7f0505b1-7b91-4390-9fd6-468b0e8922f7&amp;kclid=7f0505b1-7b91-4390-9fd6-468b0e8922f7&amp;gclid=CjwKCAjw54fdBRBbEiwAW28S9sAKKhrHWzpb6aTbi4bOTQiwt6dy6WvLDGMN5IDvVVJWvS0hC-ItIhoCvVkQAvD_BwE

After it’s heated, you could remove any bumps or twists in the frame before you reinstall it. (You may need to drill a few extra holes so you can add a some screws to keep it aligned until it cools.)

To keep the threshold watertight, I’d install flashing under the threshold and set it in sealant. 

BTW, don’t over heat the frame or it will flop around like a piece of spaghetti and you’ll never get it back to it’s original shape.