You can do that as follows:
% Firstly some sample data
T = array2timetable(randi(3,10,5),'Rowtimes',datetime() + seconds(0:9))
T = 10×5 timetable
Time Var1 Var2 Var3 Var4 Var5
____________________ ____ ____ ____ ____ ____
01-Feb-2022 16:14:43 2 2 1 1 1
01-Feb-2022 16:14:44 2 1 2 3 3
01-Feb-2022 16:14:45 3 2 2 2 3
01-Feb-2022 16:14:46 1 3 1 3 2
01-Feb-2022 16:14:47 1 3 2 2 3
01-Feb-2022 16:14:48 1 3 3 3 2
01-Feb-2022 16:14:49 3 3 1 1 1
01-Feb-2022 16:14:50 1 3 3 1 3
01-Feb-2022 16:14:51 2 2 1 1 3
01-Feb-2022 16:14:52 1 1 1 3 2
% You can do it using column numbers
T{T.Var1==1,[2 4]} = nan
T = 10×5 timetable
Time Var1 Var2 Var3 Var4 Var5
____________________ ____ ____ ____ ____ ____
01-Feb-2022 16:14:43 2 2 1 1 1
01-Feb-2022 16:14:44 2 1 2 3 3
01-Feb-2022 16:14:45 3 2 2 2 3
01-Feb-2022 16:14:46 1 NaN 1 NaN 2
01-Feb-2022 16:14:47 1 NaN 2 NaN 3
01-Feb-2022 16:14:48 1 NaN 3 NaN 2
01-Feb-2022 16:14:49 3 3 1 1 1
01-Feb-2022 16:14:50 1 NaN 3 NaN 3
01-Feb-2022 16:14:51 2 2 1 1 3
01-Feb-2022 16:14:52 1 NaN 1 NaN 2
% or using variable names
T{T.Var1==1,["Var3", "Var5"]} = nan
T = 10×5 timetable
Time Var1 Var2 Var3 Var4 Var5
____________________ ____ ____ ____ ____ ____
01-Feb-2022 16:14:43 2 2 1 1 1
01-Feb-2022 16:14:44 2 1 2 3 3
01-Feb-2022 16:14:45 3 2 2 2 3
01-Feb-2022 16:14:46 1 NaN NaN NaN NaN
01-Feb-2022 16:14:47 1 NaN NaN NaN NaN
01-Feb-2022 16:14:48 1 NaN NaN NaN NaN
01-Feb-2022 16:14:49 3 3 1 1 1
01-Feb-2022 16:14:50 1 NaN NaN NaN NaN
01-Feb-2022 16:14:51 2 2 1 1 3
01-Feb-2022 16:14:52 1 NaN NaN NaN NaN