The manual can be found here: https://www.dyson.com/medialibrary/Files/Dyson/Support/downloads/US/Floorcare/manuals/dc17_opmanual_usa%20pdf.pdf
Per page 9 (and this is consistent with every Dyson manual I've seen for uprights), you put your foot on the lower assembly and pull back the upper one. If it's really hard to pivot, even after you get it back, I'd start with just doing so with the vacuum off and move it through it's full range of motion several times to see if it eases up. Also check that the lower spot where the accessory-tool hose clicks into the lower part of the unit if completely set into place. Some manuals have mentioned this as part of troubleshooting.
Answers & Comments
The manual can be found here:
https://www.dyson.com/medialibrary/Files/Dyson/Support/downloads/US/Floorcare/manuals/dc17_opmanual_usa%20pdf.pdf
Per page 9 (and this is consistent with every Dyson manual I've seen for uprights), you put your foot on the lower assembly and pull back the upper one. If it's really hard to pivot, even after you get it back, I'd start with just doing so with the vacuum off and move it through it's full range of motion several times to see if it eases up. Also check that the lower spot where the accessory-tool hose clicks into the lower part of the unit if completely set into place. Some manuals have mentioned this as part of troubleshooting.
Good Luck!