I'm new to cycling. For the first couple of weeks of my commute all my weight was on my rear, giving me a sore backside. But then I learned the correct seating position (slight bend in the knee at pedal's lowest position), raised my seat, and 'hey presto!' no more sore backside and no sore knees. However, I now have sore shoulders and neck, and if I ride for a long enough period (30-45 minutes) without stopping, I get pins and needles in my hands. I understand this might be because my handlebars are too low. The thing is, I don't think it's possible to raise the bars. Short of buying a better-fitting bike, how can I resolve this? Is it easy to replace the stem so I can raise the bars?
The quickest answer is, Yes, it's easy to change the stem, but before you do make sure your bar clamp is at the top of the steerer stack. If there are any spacers between the top cap and the stem clamp, then undo the top cap, then loosen the clamp bolts, slide the spacer/s off, then the stem clamp. Replace the spacers first, then the clamp.
You'll need to carefully reset the bearing load with the top bolt. To do this spin the cap bolt in finger tight, then half a turn at a time tighten the bolt with an allen key, feeling how the bearings are running as you do. The head should turn smoothly without being notchy. Then align and tighten the stem clamp, then add a quarter turn to the top cap to make sure it stays in place.
Are they riser bars? Could you rotate them in the bar clamp to give a raised hold position? You'll need to move the brake and gear levers into a hand-friendly position if you do rotate the bars.
Last resort, yes, you can get angled stems, of varying lengths. Check On_One or Planet X websites, they had a sale on their Forged 3D stems which have varying angles and lengths.