Adjust a 868MHz PCB monopole antenna to work better at 915MHz
You should ideally scale everything by 870/915 (even including the dielectric thickness...)

But as you have already built one, and have a network analyser, just trim down L4 until it resonates where you want it to.

Make a cut across the end of the track with a sharp knife, to shorten it by about 1 mm at a time. Tin the offcut and heat it with a soldering iron, after a few seconds it will lift off easily. Go slowly, don't try to do it all at once. When you've gone too far, a bit of self-adhesive copper tape or an offcut can be used to extend it again.
Measure L4 when it resonates, and make that your new design.

Of course you change the shape of the antenna like this, so its impedance at resonance will be different, but the impact on the ultimate reflection coefficient will be small.

The environment around the antenna will also have a big effect on tuning. Tune it with the final plastic housing closed over the antenna. Use the final groundplane size, and the final bare PCB size, not just the sizes from the reference antenna design. Solder mask will also change the resonant frequency slightly.

One last thought - if you have additional space, the best thing is to spread the antenna out more in the vertical direction, and then re-tune it. The closer it gets to being a simple fat 1/4 wave monopole, the better its efficiency - less PCB loss - and the wider its bandwidth, so more room for environmental effects.