Jitter in 'ppm' and 'ns'
How it can be converted from one unit to another?


You can't. The two specs are about completely different things.

When frequency tolerance is say 100ppm, then your 1MHz clock will have a frequency in the 1 000 000 Hz +/- 100 Hz. 

This says nothing about jitter. Frequency is only the average number of clock cycles in a second. Jitter is about the variance of the clock period. So, if your clock period varies randomly by Tj=10ns RMS (for example) but the period is still 1µs on average, then you can have a very accurate (but jittery) 1MHz clock.

EDIT: ...however if the accuracy is given in µs/minute (for example) then it is accuracy and not jitter. The way to convert it is quite simple...

\$ ppm = 10^6 \frac{drift}{interval} \$

so, 1µs drift over 1 minute = \$ 1µs/60s\$ = 0.016ppm (that's gotta be expensive...) it doesn't depend on the frequency, since it is a drift measured in time, over a time interval. If you multiply both by the frequency, then it is a drift measured in number of periods, over an interval which is also measured in number of periods. Frequency appears both as numerator and denominator, so it disappears.

Now about jitter, since I dont know whether you're talking about a jitter spec or a drift spec... The ppm spec is useful when you are interested about frequency accuracy. Jitter spec (or more accurately phase noise) is about spectral purity which is very important if you use the frequency as a RF carrier, also it is has a strong influence on ADC/DAC noise floors, etc. For example in data transmission (like USB) frequency accuracy is not important since the receiver synchronizes itself to the transmitter by using a PLL, but jitter in the recovered clock is extremely important, as you want to sample the received bits at the proper time.