Inverse MixColumns Operation of AES
MixColumns implements a matrix multiplication, so InvMixColumns is the multiplication by the inverse of that matrix. Thus, it boils down to computing the inverse of the matrix. It can be tedious to do by hand, but it involves only straightforward computations. Alternatively, look up the AES standard itself (FIPS 197): the inverse matrix is spelled out explicitly in section 5.3.3.

More generally, FIPS 197 is quite readable (as standards go) and if you want to know how AES works, it is recommended to read it through (and in particular section 4, that gives some details on the underlying mathematics).