I have Felt MTB Nine 60 2017 and I want to change my rear hub but don't know how to look for compatibility. It says 6 bolt QR x 135.
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135 means 135mm rear drop-out spacing - the older standardard for MTBs that is still used on less expensive bikes.
QR is quick release, as opposed to a solid bolt on axle or more modern through axle.
6-bolt refers to the disc mounting standard, the other being Shimano's Center-Lock.
The free hub body width needs to be compatible with 8/9/10 speeds cassettes. You'll obviously need a hub with the same spoke count.