How to change the frequency of this sawtooth generator circuit?
1) The easiest way is by varying the 39k emitter resistor. Calling this resistor Rt, the approximate formula is $$i=\frac {15 -\frac{27k\times 15}{27k + 120k}-0.7}{R_t}  $$ and $$f = \frac{i}{C\times \frac{15}{3}} $$

2) No. Or, at least, not easily. If you run the 555 from +10 and -5 volts instead of +15 and ground, the bottom of the sawtooth will be approximately at 0 volts. Or, you can use an operational amplifier to shift your voltage levels by 5 volts, but this will also require using a negative power supply.