  In this paper, we study six slowly rotating mid-to-late M~dwarfs (rotation
period $P_{\mathrm{rot}} \approx 40-190\,\mathrm{dy}$) by analysing
spectropolarimetric data collected with SPIRou at the Canada-France-Hawaii
Telescope as part of the SPIRou Legacy Survey from 2019 to 2022. From
$\approx$100--200 Least-Squares-Deconvolved (LSD) profiles of circularly
polarised spectra of each star, we confirm the stellar rotation periods of the
six M~dwarfs and explore their large-scale magnetic field topology and its
evolution with time using both the method based on Principal Component Analysis
(PCA) proposed recently and Zeeman-Doppler Imaging. All M~dwarfs show
large-scale field variations on the time-scale of their rotation periods,
directly seen from the circularly polarised LSD profiles using the PCA method.
We detect a magnetic polarity reversal for the fully-convective M~dwarf
GJ~1151, and a possible inversion in progress for Gl~905. The four
fully-convective M~dwarfs of our small sample (Gl~905, GJ~1289, GJ~1151,
GJ~1286) show a larger amount of temporal variations (mainly in field strength
and axisymmetry) than the two partly-convective ones (Gl~617B, Gl~408).
Surprisingly, the six M~dwarfs show large-scale field strengths in the range
between 20 to 200\,G similar to those of M~dwarfs rotating significantly
faster. Our findings imply that the large-scale fields of very slowly rotating
M~dwarfs are likely generated through dynamo processes operating in a different
regime than those of the faster rotators that have been magnetically
characterized so far.
